336 



NATURE 



[November io, 1921 



It is agreed that fluctuations blend. The following 

 then are the rival suppositions :— (a) That the in- 

 heritance of Mendelian characters is alternate, and 

 (6) that only the reproduction of them is alternate ; 

 (a) that Mendelian characters do not blend, and (/?) 

 that they do blend (not with the alternate character, 

 but with the similar character, latent or patent in 

 the mate) ; (a) that Mendelian characters are stable 

 and can be eliminated only by selective breeding or 

 by retrogressive mutations, and (b) that they fluc- 

 tuate, and therefore are liable, when useless, to 

 undergo retrogression like other useless characters ; 

 (a) that the function of sex is to mix characters 

 as marbles of two colours are mixed in a bag, and 

 (6) that the function of sex is to blend characters 

 as two paints are blended on a palette (apparently the 

 result of this blending is retrogressive through a pre- 

 ponderance of retrogressive variations ; hence the 

 decrease and ultimate disappearance of useless 

 characters, including mutations, whereby the race is 

 stripped of redundancies as an athlete is stripped for 

 a race ; in this way, presumably, the life-history of 

 the race has been shortened until recapitulation in 

 development is possible) ; (a) that species and varieties 

 have all originated through mutations, and (b) that all 

 natural species and varieties •'ave originated and 

 evolved through the selection of fluctuations, while 

 the selection of mutations has played a great part 

 in the creation of artificial varieties ; and finally (a) 

 that like does not necessarily beget like when parent 

 and offspring develop under like conditions (for if the 

 doctrine of the Mendelian inheritance be true, the pure 

 extracted recessive is by nature unlike the impure 

 dominant parent whence it is derived), and (b) that 

 apart from variations, like does exactly and neces- 

 sarily beget like under like conditions of nurture — 

 for example, a daughter would have developed male 

 traits like her father had her nurture been similar. 



Now let us seek crucial examples. Their dis- 

 covery should not be difficult among the vast col- 

 lections of facts which have been recorded about living 

 beings. If the inheritance (not merely the reproduc- 

 tion) of Mendelian traits is alternate, how is it possible 

 to account for the oft-observed reappearance in purely 

 bred domesticated varieties of long-lost ancestral 

 characters ? According to Mendelian theory such 

 traits should have been totally eliminated perhaps 

 hundreds, or even thousands, of generations before 

 from the "pure" dominants and recessives that are 

 supposed to have carried on the heritage. As Darwin 

 noted : " We see that in purely bred races [of pigeons] 

 of every kind known in Europe, blue birds occa- 

 sionally appear having all the marks which charac- 

 terise C. livia.^^ "Purely bred Game, Malay, Cochin, 

 Dorking, Bantam . . . and silk fowls may frequently 

 be met which are almost identical in plumage 

 with the wild G. bankiva.^^ "Pure" extracted reces- 

 sives have given dominant offspring, and vice versa. 

 Many similar cases might be cited. If "the central 

 phenomenon of Mendelian heredity is segregation," 

 not even the reunion of disunited factors and deter- 

 miners can account for these reversions ; for if the 

 breeding has been pure, then reunion, ex hypothesi, 

 is inconceivable. In parenthesis, it may be noted that 

 an individual may resemble a remote ancestor in two 

 ways : — (i) He may reproduce a dormant ancestral 

 trait, thereby rendering latent the alternate character 

 which had been patent in his immediate predecessors ; 

 or (2) he may, through a failure in nature or nurture, 

 fail to recapitulate the life -history of his race beyond 

 the point reached by an ancestor and by himself as a 

 foetus— for example, " Occasionally a foal is born with 

 two hoofs on one or more of its limbs ; at long in- 

 tervals a foal appears with three hoofs on one or 

 more of its limbs." 



NO. 2715, VOL. 108] 



"The fact that the gametes of the cross transmit 

 [or reproduce] each member of the pair pure is as 

 strong an indication as can be desired of the discon- 

 tinuuy between them." If that be so, the converse 

 must be true also — the fact that the gametes of the 

 cross transmit each member of the cross blended is 

 as strong an indication as can be desired of the con- 

 tinuity between them. In other words, if offspring 

 reproduce divergent parents unblended, then there 

 is a presumption that one or other, or both, of each 

 pair of differences arose through mutation ; but if 

 there; is blending the presumption is that the unlike- 

 nesses arose through fluctuation, it may be through 

 the accumulation of many successive fluctuations. 

 For example, since in the crossing of white and black 

 human races eye-colour does not blend, it is probable 

 that this particular unlikeness arose through muta- 

 tion (and, since British eyes are of many colours, 

 that we are a very hybrid race). On the other hand, 

 since skin-colour blends, it is probable that this colour 

 difference arose through the accumulation of fluctua- 

 tions. Similarly, if the progeny of racehorses crossed 

 with ordinary horses do not blend hair-colour, but 

 are intermediate as regards speed, the presumption is 

 that the divergency in hair-colour arose through 

 mutation, but that in speed evolved through fluctua- 

 tions. Now comes the point I am driving at. When 

 we cross artificial varieties we frequently discover 

 instances of alternate reproduction, especially in traits 

 which are not vital for existence. But when we cross 

 natural varieties {which carry few superfluities) we 

 almost invariably get blending in nearly everything 

 except sex and characters associated with sex (for 

 example, human eye-colour, which is an attraction). 

 A few years ago there was a beautiful example of 

 blending in the London Zoo in a cross between brown 

 and polar bear, recrossed with the latter. Essentially 

 it was a case of mulatto and quadroon. 



But the crossings of natural and artificial varieties 

 reveal another significant difference. Darwin wrote : 

 ■' Besides visible changes which it [the germ] under- 

 goes, we must believe it crowded with invisible 

 characters proper to both sexes, to both the right 

 and left sides of the body, and to a long line of male 

 and feniale ancestors separated bv hundreds, and even 

 thousands, of generations from the present time ; 

 and these characters, like those written on paper in 

 invisible ink, lie ready to be evolved whenever the 

 organism is disturbed by certain unknown conditions. 

 . . . We conclude that a tendency to this peculiar 

 form of transmission is an integral part of the general 

 law of inheritance." But if we study the evidence 

 which Darwin cites and which led him to this con- 

 clusion, it becomes plain that he relied altogether on 

 facts derived from artificial varieties. So far as I 

 know (with one doubtful exception, Kalenchoe 

 flainmea crossed at Kew with K. Bentii), there has 

 not been recorded a single instance of the reappear- 

 ance of a latent ancestral trait when natural varieties 

 have been crossed; but the crossing of artificial varie- 

 ties has revealed multitudes of them. Often they have 

 appeared even when there is no crossing, as in 

 nigeons, fowls, and many plants — hence De Vries's 

 " ever-soorting " varieties. 



Of all the foregoing the human race affords excel- 

 lent illustrations. Man is a natural species, divided 

 into a multitude of natural varieties. Where there 

 has been perpetual w'ar, accompanied by the killing 

 of the vanquished, as in Papua, almost every valley 

 and island has its biologicallv distinct race. Obviously, 

 lack of intercourse Hnterbreeding) swiftly produces 

 racial divergence, which steadilv increases generation 

 after generation — as witness the great unlikenesses 

 between races separated by great distances, and there- 

 fore long durations of time. Human written history 



