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MENDELISM
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MENDELISM
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To define what some biologists See also:call Mendelism briefly is not possible
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Within See also:recent years there has come to biologists a new See also:idea of the nature of living things, a new conception of their potentialities and of their limitations; and for this we are primarily indebted to the See also:work of Gregor Mendel
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See also:Peasant boy, See also: Thus on crossing a tall with a See also:dwarf and paying attention to this pair of characters alone, he found that the hybrids (or F1 See also:generation) were all tall and that no intermediates appeared . Accordingly he termed the tall See also:character dominant and the dwarf character recessive . On allowing these hybrids to fertilize themselves in the See also:ordinary way he obtained a further generation which on the See also:average was composed of three tails to one dwarf . Subsequent experi- ment showed that the T x D P dwarfs always bred true, as did also one out of every three tails; the two remaining tails behaved as the See also:original hybrids in D .. Fs giving three tails to one dwarf . Having regard to D F the characters, tallness { 3 and dwarfness, three and only three kinds of peas exist, viz. dwarfs which breed true, tails which breed true, and tails which give a fixed proportion of tails and dwarfs . The relation between these three forms may be briefly summarized in the subjoined See also:scheme, in which pure tall and dwarf are represented by T and D respectively, while [T] denotes the tails which do not breed true . Experiments were also made with several other pairs of characters, and the same mode of inheritance was shown to hold See also:good throughout . Unit-Characters.—As Mendel clearly perceived, these definite results See also:lead inevitably to a precise conception of the consti- tution of the reproductive cells, or gametes; and to appreciate fully the See also:change wrought in our point of view necessitates a brief digression into the essential features of the reproductive process . A sexual process (see See also:SEx) is almost universal among animals and See also:plants, and consists essentially of the See also:union of two gametes, of which one is produced by either See also:parent . Every gamete contains small definite bodies known as chromosomes, and the number of these is, with few known exceptions, See also:con- stant for the gametes of a given See also:species . On the See also:fusion of two gametes the resulting See also:cell or zygote has therefore a See also:double structure, for it contains an equal number of chromosomes brought in by the paternal and by the maternal gamete —in the See also:case of a plant by the See also:pollen See also:grain as well as by the ovule . By a process of re- peated See also:division the zygote gives rise to a plant (or an See also:animal) whose cells appar- ently retain the double structure throughout . Cer- See also:tain of the cells of such a zygote become the germ cells and are set apart, as it were, for the formation of gametes . See also:Histology has shown that when this occurs -the cells lose the double structure which they had hitherto possessed, and that as the result of a process known as the reduction e division gametes are formed ° in which the number of chromosomes is one See also:half of that which characterizes the cells of the zygote . It is generally acknowledged that the chromosomes See also:play an important See also:part in the hereditary process, and it is possible that the divisions which they undergo in gametogenesis are connected with the observed inheritance of characters . We shall refer later to the few observations which seem to connect the two sets of phenomena . Our conception of what occurs when a See also:cross is made between two individuals may be illustrated by the See also:diagram which forms fig . 2 . Zygotes are here represented by squares and gametes by circles . The dominant and recessive characters are indicatedby small See also:plain and See also:black rectangles . Each zygote must contain two and each gamete but one of these unit-characters . Zygotes such as the original parents which breed true to a given character are said to be homozygous for that character, and from their nature such homozygotes must produce identical gametes . Consequently when a cross is made only one See also:kind of zygote can be formed, viz. that containing both the dominant and recessive unit-characters . When the germ-cells of such a heterozygote split to See also:form gametes, these, as indicated in fig . 2, will be of two sorts containing the dominant and recessive characters respectively, and will be produced in equal See also:numbers . If we are dealing with a hermaphrodite plant such as the pea the ovules will consist of one half bearing only the dominant character and one half bearing only the recessive character; and this will be true also of the pollen grains . Consequently each dominant ovule has an equal See also:chance of being fertilized by a dominant or by a recessive pollen grain, and the dominant ovules must therefore give rise to equal numbers of dominant homozygous and of heterozygous plants . Similarly the recessive ovules must give rise to equal numbers of recessive homozygotes and of heterozygotes . Hence of the See also:total offspring of such a plant one See also:quarter will be pure dominants, one quarter recessives, and one half heterozygotes as indicated in fig . 2 . Where one character is completely dominant over the other, heterozygotes will be indistinguishable in See also:appearance from the homozygous dominant, and the Fs generation will be composed of three plants of the dominant form to each recessive . These are the proportions actually found by Mendel in the pea and by many other more recent observers in a number of plants and animals . The experimental facts are in accordance with the conception of unit characters and their transmission from zygote to gamete in the way outlined above; and the numerical results of breeding experiments are to be regarded as proving that in the formation of gametes from the heterozygote the unit-characters are treated as unblending entities separating cleanly, or segregating, from one another . From this it follows that any gamete can carry but one of a pair of unit-characters and must therefore be pure for that character . The principle of the segregation of characters in gametogenesis with its natural corollary, the purity of the gametes, is the essential part of Mendel's discoveries . The quite distinct phenomenon of dominance observed by him in Pisum occurs in many other cases, but, as will appear below, is by no means universal . Illustrations.—Mendelian inheritance in its simplest form, i.e. for a single pair of characters, has already been shown to occur in many species of animals and plants, and for many very diverse characters . In some cases See also:complete dominance of one of the pair of unit-characters occurs; in others the form of heterozygote is more or less intermediate . Fresh cases are continually being recorded and the following See also:short See also:list can but serve to give some idea of the variety of characters in which Mendelian inheritance has been demonstrated . A . Dominance. neatly or quite complete . (The dominant character is given first) . Tall and dwarf See also:habit (pea, sweet pea) . See also:Round See also:seed and wrinkled seed (pea) . See also:Long pollen and round pollen (sweet pea) . See also:Starch and See also:sugar endosperm (See also:maize) . Hoariness and See also:absence of hairs (See also:stocks, Lychnis) .
Beardless and bearded See also:condition (See also:wheat)
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Prickliness and smoothness of fruits (Datura)
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See also:Palm and See also:fern See also:leaf (Primula)
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See also:Purple and red See also:flowers (sweet pea, stocks, &c.)
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Fertility and sterility of anthers (sweet pea)
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Susceptibility and See also:immunity to See also:rust (wheat)
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See also:Rose See also:comb and single comb (fowls)
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Black and See also: Short and long " See also:Angora " coat (rabbits) . Normal and waltzing habit (mice) . Deformed See also:hand with but two phalanges in digits and normal hand (man) . ----------------i T [X] [T] T T [-II LT . D Parents .0 Ft Zygote / \ ' Fa Zygotst FIG . 2 . B . Absence of dominance, the heterozygote being more or less intermediate in form . Black and white splashed plumage (Andalusian fowls) . Lax and dense ears (wheat) . Six rowed and two rowed ears (See also:barley) . Dominance.—The meaning of this phenomenon is at See also:present obscure, and we can make no See also:suggestion as to why it should be complete in one case, partial in another, and entirely absent in a third . When found it is as a See also:rule definite and orderly, but there are cases known where irregularity exists . The extra toe characteristic of certain breeds of fowls, such as Dorkings, behaves generally as a dominant character, but in certain cases it has been ascertained that a.See also:fowl without an extra toe may yet carry the extra toe character . It is possible that in 1lli some cases dominance may be conditioned by the presence of other features, and certain crosses in See also:sheep lend colour to the supposition that sex may be such a feature . A cross between the polled See also:Suffolk and the horned See also:Dorset breeds results in horned rams and polled ewes only, though in the See also:F2 generation both sexes appear with and without horns . At present the simplest See also:hypothesis which fits the facts is that horns are dominant in the male and recessive in the See also:female . It is important not to confuse cases of apparent reversal of dominance such as the above with cases in which a given visible character may be the result of two entirely different causes . One white See also:hen may give only colour chicks by a coloured See also:cock, whilst the same cock with another white hen, indistinguishable in appearance from the former, will give only white chickens containing a few dark See also:ticks . There is here no reversal of dominance, but, as has been abundantly proved by experiment, there are two entirely distinct classes of white fowls, of which one is dominant and the other recessive to colour . The Presence and Absence Hypothesis.—Whether the phenomenon of dominance occur or not, the unit-characters exist in pairs, of which the members are seemingly interchangeable . In virtue of this behaviour the unit-characters forming such a pair have been termed allelomorphic to one another, and the question arises as to what is the nature of the relation between two allelomorphs . The fact that such cases of heredity as have been fully worked out can all be formulated in terms of allelomorphic pairs is suggestive, and has led to what may be called the " presence and absence " hypothesis . An allelomorphic pair represents the only two possible states of any given unit-character in its relation to the gamete, viz. its presence or its absence . When the unit-character is present the quality for which it stands is manifested in the zygote: when it is absent some other quality previously concealed is able to appear . When the unit-character for yellowness is present in a pea the seeds are yellow, when it is absent the seeds are See also:green . The green character is underlying in all yellow seeds, but can only appear in the absence of the unit-character for yellowness, and greenness is allelomorphic to yellowness because it is the expression of absence of yellowness . Dihybridism.—The instances hitherto considered are all See also:simple cases in which the individuals crossed differ only in one pair of unit-characters . Mendel himself worked out cases in which the parents differed in more than one allelomorphic pair, and he pointed out that the principles involved were capable of indefinite extension . The inheritance of the various allelomorphic pairs is to be regarded as entirely See also:independent . For example, when two individuals AA and aa are crossed the See also:composition of the F2 generation must be AA + 2Aa +aa . If we suppose that the two parents differ also in the allelomorphic pair B–b, the composition of the F2 generation for this pair will be BB + 2Bb + bb . Hence of the zygotes which are homozygous for AA one quarter will carry also BB, one quarter bb, and one half Bb . And similarly for the zygotes which carry Aa or aa . The various combinations possible together with the relative frequencies of their occurrence may be gathered from fig . 3 . Of the 16 zygotes there are: 9 containing A and B 3 containing B but not A 3 „ A but not B If neither A nor B In a case of dihybridism the Fl zygote must be heterozygous for the two allelomorphic pairs, i.e. must be of the constitution Aa Bb . It is obvious that such a result may be produced in two ways, either by the union of two gametes, Ab and aB, or of two gametes AB and ab . In the former case each parent must be homozygous for one dominant and one recessive character; in the latter case one parent must be homozygous for both the dominant and the other for both recessive characters . The results of a cross involving. dihybridism may be complicated in several ways by the reaction upon one another of the unit-characters belonging to the See also:separate allelomorphic pairs, and it will be convenient to consider the various possibilities apart . r . The simplest case is that in which the two allelomorphic pairs affect entirely distinct characters . In the pea tallness is dominant to dwarfness and yellow seeds are dominant to green . . When a yellow tall is crossed with a green dwarf the Fl generation consists entirely of tall yellows . Precisely the same result is obtained by crossing a tall green with a dwarf yellow . In either case all the four characters involved are visible in one or other of the parents . Of every i6 plants produced by the tall yellow Fl, co are tall yellows, 3 are tall greens, 3 are dwarf yellows, and r is a dwarf green . If we denote the tall and dwarf characters by A and a, and the yellow See also:WALNUT FIG.4 . SI IV OL.E The four types of comb referred to in the See also:text are shown here . All the drawings were made from male birds . . In the hens the combs are smaller . All four types of comb are liable to a certain amount of See also:minor variation, and the walnut especially so . The presence of See also:minute bristles on its posterior portion, however, serves at once to distinguish it from any other comb . and green seed characters by B and b respectively, then the , constitution of the F2 generation can be readily gathered from fig . 3 . 2 . When the two allelomorphic pairs affect the same structure we may get the phenomenon of novelties appearing in Fl and F2 . Certain breeds of fowls have a " rose " and others a " pea y' comb (fig . 4) . On crossing the two a " walnut " comb results, and the offspring of such walnuts bred together consist of 9 walnuts, 3 See also:roses, 3 peas, and I single comb in every 16 birds . This case may be brought into See also:line with the scheme in fig . 3 if we consider the allelomorphic pairs concerned to AA AA Aa As .. BB Bb BB Bb SB bb bB bb aA aA as as BB Bb BB Bb bB b b bb bB b P E A be rose (A) and absence of rose (a), and pea (B) and absence of pea (b) . The zygotic constitution of a rose is therefore AAbb, and of a pea aaBB . A zygote containing both rose and pea is a walnut: a zygote containing neither rose nor pea is a single . The See also:peculiar feature of such a case lies in the fact that absence of rose and absence of pea are the same thing, i.e. single; and this is doubtless owing to the fact that the characters rose and pea both affect the same structure, the comb . 3 . Cases exist in which the characters due to one allelomorphic pair can only become See also:manifest in the presence of a particular member of the other pair . If in fig . 3 the characters due to B–b can only manifest themselves in the presence of A, it is obvious that this can happen in twelve cases out of sixteen, but not in the remaining four, which are homozygous for aa . An example of this is to be found in the inheritance of coat colour in rabbits, rats and mice where the allelomorphic pairs concerned are See also:wild grey colour (B) dominant to black (b) and pigmentation (A) dominant to albinism (a) . Certain albinos (aaBB) crossed with blacks (AAbb) give only greys (AaBb), and when these are bred together they give 9 greys, 3 blacks and 4 albinos . Of the 4 albinos 3 carry the grey character and r does not, but in the absence of the pigmentation See also:factor (A) this is not visible . The ratio 9 : 3 : 4 must be regarded as a 9 : 3 : 3 : I ratio, in which the last two terms are visibly indistinguishable owing to the impossibility of telling by the See also:eye whether an See also:albino carries the character for grey' or not . 4 . The appearance of a zygotic character may depend upon the coexistence in the zygote of two unit-characters belonging to different allelomorphic pairs . If in the scheme shown in fig . 3 the manifestation of a given character depends upon the simultaneous presence of A and B, it is obvious that 9 of the 16 zygotes will present this character, whilst the remaining 7 will be without it . This is shown graphically in fig . 5, where the 9 squares have been shaded and the 7 See also:left plain . The sweet pea offers an example of this phenomenon . White sweet peas breed true to whiteness, but when certain strains of whites are crossed the offspring are all coloured .. In the next generation (F2) these F1 plants give rise to 9 coloured and 7 whites in every 16 plants . Colour here is a See also:compound character whose manifestation depends upon the co-existence of two factors in the zygote, and each of the original parents was homozygous for one of the two factors necessary to the See also:production of colour . The ratio 9 : 7 is in reality a 9 : 3 : 3 : I ratio in which, owing to See also:special conditions,- the zygotes represented by the last three terms are indistinguishable from one another by the eye . The phenomena of dihybridism, as illustrated by the four examples given above, have been worked out in many other cases for plants and animals . Emphasis must be laid upon the fact that, although the unit-characters belonging to two pairs may react upon one another in the zygote and affect its character, their inheritance is yet entirely independent . Neither grey nor black can appear in the See also:rabbit unless the pigmentation factor is also present; nevertheless, gametic segregation of this pair of characters takes See also:place in the normal way among albino rabbits, though its effects are never visible until a suitable cross is made . In cases of trihybridism the Mendelian ratio for the forms appearing in F2 is 27 : 9 :.9 : 9: 3 : 3 : 3 : I, i.e . 27 showing dominance of three characters, three See also:groups of 9 each showing dominance of two characters, three groups of 3 each showing dominance of one character, and a single individual out of 64 which is homozygous for all three recessive characters . It is obvious that the See also:system can be indefinitely extended to embrace any number of allelomorphic pairs . Reversion.—Facts such as those just dealt with in connexion with certain cases of dihybridism throw an entirely new See also:light upon the phenomenon known as reversion on crossing . This is now seen to consist in the See also:meeting of factors which had in some way or other become separated in phylogeny . The albino rabbit when crossed with the black " reverts " to the wild grey colour, because each parent supplies one of the two factors upon which the manifestation of the wild colour depends . So also the wild purple sweet pea may come as a reversion on crossing two whites . In such cases the reversion appears in the Ft generation, because the two factors upon which it depends are the dominants of their respective allelomorphic pairs . Where the reversion depends upon the simultaneous absence of two characters it cannot appear until the F2 generation . When fowls with rose and pea combs are crossed the reversionary single comb characteristic of the wild See also:Gallus bankiva first appears in the F2 generation . Gametic Coupling.—In certain cases the See also:distribution of characters in heredity is complicated by the fact that particular unit-characters tend to become associated or coupled together during gametogenesis . In no case have we yet a complete explanation of the phenomenon, but in view of the important bearing which these facts must eventually have on our ideas of the gametogenic process an See also:illustration may be given . The case in which two white sweet peas gave a coloured on crossing has already been described, and it was seen that the production of colour was dependent upon the meeting of two factors, of which one was brought in by each parent . If the allelomorphic pairs be denoted by C–c and R–r, then the zygotic constitution of the two parents must have been CCrr and ccRR respectively . The F1 plant may be either purple or red, two characters which form an allelomorphic pair in which the former is dominant, and which may be denoted by the letters B–b . If B is brought in by one parent only the F1 plant will be heterozygous for all three allelomorphic pairs, and therefore of the constitution Cc Rr Bb . In the F2 generation the ratio of coloured to white must be 9 : 7, and of purple to red 3 I; and experiment has shown that this generation is composed on the average of 27 purples, 9 reds and 28 whites out of every 64 plants . The exact composition of such a See also:family may be gathered from the accompanying table (fig . 6) . So far the case is perfectly smooth, and it is only on the introduction of another character that the phenomenon of partial coupling is witnessed . Two kinds of pollen grain occur in the sweet pea . In some plants they are oblong in shape, whilst in others they are round, the latter condition being recessive to the former . If the original white parents were homozygous for long and round respectively the Fl purple must be heterozygous, and in the F2 generation, as experiment has shown, the ratio of longs to rounds for the whole family is 3 : I . But among the purples there are about twelve longs to each round, the excess of longs here being balanced by the reds, where the proportion is t long to about 3.5 rounds . There is partial coupling of long pollen with the purple colour and a complementary coupling of the red colour with round pollen . This result would be brought about if it were supposed that seven out of every eight purple gametes produced by the Fl plant carried the long pollen character, and that seven out of every eight red gametes carried the round pollen character . The facts observed See also:fit in with the supposition that the gametes are produced in See also:series of sixteen, but how such result could be brought about is a question which for the present must remain open . See also:Spurious Allelomorphism.—Instances of association between characters are known in which the connection is between the dominant member of one pair and the recessive of another . In many sweet peas the See also:standard folds over towards the wings, and the See also:flower is said to be hooded . This " hooding " behaves as a recessive towards the erect standard . Red See also:sap colour is also recessive to purple . In families where purples and reds as well as erect and hooded See also:standards occur it has been found, as might be expected, that erect standards are to hooded ones, and that purples are to reds as 3:1 . Were the case one of simple dihybridism the F2 generation should be composed of q erect purples, 3 hooded purples, 3 erect reds and i hooded red in every 16 . Actually it is composed of 8 erect purples, 4 hooded purples and 4 erect reds . The See also:hood will not See also:associate with the red, but occurs only on the purples . Cases like this are best interpreted on the See also:assumption that during gametogenesis there is some form of repulsion between the members of the different pairs—in the present instance between the factor for purple and that for the erect standard—so that all the gametes which contain the purple factor are See also:free from the factor for the erect standard . To the process involved in this assumption the See also:term spurious allelomorphism has been applied . Sex.—On the existing evidence it is probable that the inheritance of sex runs upon the same determinate lines as that of other characters . Indeed, there occurs in the sweet pea what may be regarded as an instance of sex inheritance of the simplest kind . Most sweet peas are hermaphrodite, but some are found in which the anthers are sterile and the plants See also:function only as See also:females . This latter condition is recessive to the hermaphrodite one and segregates from it in the ordinary way . Most cases of sex inheritance, however, are complicated, and it is further possible that the phenomena may be of a different See also:order in plants and animals . Instructive in this connexion are certain cases in which one of the characters of an allelomorphic pair may be coupled with a particular sex . The See also:pale lacticolor variety of the See also:currant See also:moth (See also:Abraxas grossulariata) is recessive to, the normal form, and in families produced by heterozygous parents one quarter of the offspring are of the variety . Though the sexes occur in approximately equal numbers, all the lacticolor its such families are females; and the association of sex with a character exhibiting normal segregation is strongly suggestive of a similar process obtaining for sex also . See also:Castle has worked out similar cases in other See also:Lepidoptera and has put forward an hypothesis of sex inheritance on the basis of the Mendelian segregation of sex determinants . An ovum or spermatozoon can carry either the male or the female character, but it is essential to Castle's hypothesis that a male spermatozoon should fertilize only a female ovum and See also:vice versa, and consequently on his view all zygotes are heterozygous in respect of sex . Whether any such gametic selection as that postulated by Castle occurs here or elsewhere must for the present remain unanswered . Little evidence exists for it at present, but the possibility of its occurrence should not be ignored . More recently evidence has been brought forward by See also:Bateson and others (3) which supports the view that the inheritance of sex is on Mendelian lines . The See also:analysis of cases where there is a closer association between a Mendelian character and a particular sex has 'suggested that femaleness is here dominant to maleness, and that the latter sex is homozygous while the former is heterozygous . Chromosomes and Unit-Characters.—Breeding experiments have established the conception of definite unit-characters existing in the cells of an organism: in the cell histology has demonstrated the existence of small definite bodies—the chromosomes . During gametogenesis there takes place what many histologists regard as a differentiating division of the chromosomes: at the same See also:period occurs the segregation of the unit-characters . Is there a relation between the postulated unit-character and the visible chromosome, and if so what is this relation ? The researches of E . B .
See also: Heredity we now perceive to be a method of analysis, and the facts of heredity constitute a series of reactions which enable us to argue towards the constitution of living matter . And essential to any method of analysis is the recognition of the individuality of the individual . Constitutional See also:differences of a See also:radical nature may be concealed beneath apparent identity of See also:external form . Purple sweet peas from the same pod, indistinguishable in appearance and of identical ancestry, may yet be fundamentally different in their constitution . From one may come purples, reds and whites, from another only purples and reds, from another purples and whites alone, whilst a See also:fourth will breed true to purple . Any method of investigation which fails to take account of the radical differences in constitution which may underlie external similarity must necessarily be doomed to failure . Conversely, we realize to-day that individuals identical in constitution may yet have an entirely different ancestral See also:history . From the cross between two fowls with rose and pea combs, each of irreproachable See also:pedigree for generations, come single combs in the second generation, and these singles are precisely similar in their behaviour to singles bred from strains of unblemished ancestry . In the ancestry of the one is to be found no single over a long series of years, in the ancestry of the other nothing but singles occurred . The creature of given constitution may often be built up in many ways, but once formed it will behave like others of the same constittt tion . The one sure test of the constitution of a living thing lies in the nature of the gametes which it carries, and it is the analysis of these gametes which forms the See also:province of heredity . The clear cut and definite mode of transmission of characters first revealed by Mendel leads inevitably to the conception of a definite and clear-cut basis for those characters . Upon this structural basis, the unit-character, are grounded certain of the phenomena now termed variation . Varieties exist as such in virtue of differing in one or more unit-characters from what is conventionally termed the type; and since these unit- MENDELSSOHN, See also:MOSES (1729-1786), Jewish philosopher, was born in See also:Dessau in 1729 . His See also:father's name was Mendel, and he was later on surnamed Mendelssohn (=son of Mendel) . He was the foremost Jewish figure of the 18th See also:century, and to him is attributable the See also:renaissance of the See also:House of See also:Israel . With this third Moses (the other two being the Biblical lawgiver and Moses See also:Maimonides) a new era opens in the history of the Jewish See also:people . Mendel Dessau was a poor See also:scribe—a writer of scrolls—and his son Moses in his boyhood See also:developed curvature of the spine . His See also:early See also:education was cared for by his father and by the See also:local See also:rabbi, See also:David See also:Frankel . The latter, besides teaching him the See also:Bible and See also:Talmud, introduced to him the See also:philosophy of Maimonides (q.v.) . Frankel received a call to See also:Berlin in 1743 . Not many months later a weakly lad knocked at one of the See also:gates of Berlin . He was admitted after an altercation, and found a warm welcome at the hands of his former teacher . His See also:life at this period was a struggle against crushing poverty, but his scholarly ambition was never relaxed . A refugee See also:Pole, Zamosz, taught him See also:mathematics, and a See also:young Jewish physician was his See also:tutor in Latin . He was, however, mainly self-taught . " He learned to spell and to philosophize at the same time" (See also:Graetz) . With his scanty earnings he bought a Latin copy of See also:Locke's See also:Essay concerning the Human Understanding, and mastered it with the aid of a Latin See also:dictionary . He then made the acquaintance of See also:Aaron See also:Solomon Gumperz, who taught him the elements of See also:French and See also:English . In 1750 he was appointed by a wealthy See also:silk-See also:merchant, See also:Isaac Bernhard, as teacher to his See also:children . Mendelssohn soon won the confidence of Bernhard, who made the young student successively his See also:book-keeper and his partner . Gumperz or See also:Hess rendered a conspicuous service to Mendelssohn and to the cause of enlightenment in 1754 by introducing him to See also:Lessing . Just as the latter afterwards makes Nathan the See also:Wise and See also:Saladin meet over the See also:chess-See also:board, so did Lessing and Mendelssohn actually come together as lovers of the See also:game . The Berlin of the day—the day of See also:Frederick the See also:Great—was in a moral and intellectual ferment . Lessing was the great liberator of the See also:German mind . He had already begun his work of See also:toleration, for he had recently produced a See also:drama (See also:Die Juden, 1749), the See also:motive of which was to prove that a See also:Jew can be possessed of See also:nobility of character .
This notion was being generally ridiculed as untrue, when Lessing found in Mendelssohn the realization of his See also:dream
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Within a few months of the same See also:age, the two became See also:brothers in intellectual and See also:artistic cameraderie
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Mendelssohn owed his first introduction to the public to Lessing's admiration
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The former had written in lucid German an attack on the See also:national negle,ct of native philosophers (principally See also:Leibnitz.), and See also:lent the See also:manuscript to Lessing
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Without consulting the author, Lessing published anonymously Mendelssohn's Philosophical Conversations (Philosophische Gesprache) in 1755
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In the same year there appeared in See also:Danzig an See also:anonymous See also:satire, See also:Pope a Metaphysician (Pope ein Metaphysiker), the authorship of which soon transpired
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It was the See also:joint work of Lessing and Mendelssohn
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From this time Mendelssohn's career was one of ever-increasing brilliance
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He became (1756-1759) the leading spirit of See also:Nicolai's important See also:literary undertakings, the Bibliothek and the Literaturbriefe, and ran some See also:risk (which Frederick's good nature obviated) by somewhat freely criticizing the poems of the See also: 
In See also:October 1763 the king granted Mendelssohn the See also:privilege of Protected Jew (Schutz-See also:Jude)--which assured his right to undisturbed See also:residence in Berlin . As a result of his See also:correspondence with Abbt, Mendelssohn resolved to write on the See also:Immortality of the Soul . Materialistic views were at the time rampant and fashionable, and faith in immortality was at a See also:low ebb . At this favourable juncture appeared the Phddon (1767) . Modelled on See also:Plato's See also:dialogue characters must from their behaviour in transmission be regarded as discontinuous in their nature, it follows that the variation must be discontinuous also . A present tendency of thought is to regard the discontinuous variation or mutation as the material upon which natural selection See also:works, and to consider that the process of See also:evolution takes place by definite steps . Darwin's opposition to this view rested partly upon the idea that the discontinuous variation or See also:sport would, from the rarity of its occurrence, be unable to maintain itself against the swamping effects of intercrossing with the normal form . Mendel's work has shown that this objection is not valid, and the precision of the mode of inheritance of the discontinuous variation leads us to inquire if the small or fluctuating variation can be shown to have an equally definite physiological basis before it is admitted to play any part in the production of species . Until this has been shown it is possible to consider the discontinuous variation as the unit in all evolutionary change, and to regard the fluctuating variation as the uninherited effect of environmental See also:accident . The Human Aspect.—In conclusion we may briefly allude to certain See also:practical aspects of Mendel's See |