January 29, 1920] 



NATURE 



567 



direction. Whitman's most cogent evidence is 

 derived from a knowledge of the juvenal plumage 

 in many wild species, and naturalists can scarcely 

 fail to agree with his interpretation on this point, 

 for he shows that many species which have more 

 or less completely lost their chequers or bars in 

 the adult plumage pass through a stage in which 

 these markings appear. 



(2) This brings out another aspect of ^^'h^t- 

 man's work — his strong support of the recapitula- 

 tion theory. In his own words, "all development 

 . . . is essentially a repeating or recapitulating 

 process. This is the central fact of heredity and 

 the doctrine of descent." Elsewhere he refers to 

 heredity as "nature's silent rehearsal of past 

 history." In this connection he pointed out that 

 the formula, "ontogeny recapitulates phylogeny," 

 places the emphasis in the wrong place, since 

 phylogeny can be nothing more than the lineal 

 sequences of ontogeny regarded from the historical 

 point of view, while recapitulation is simply repro- 

 ductive repetition. The orthogenetic process is 

 considered to be the primary and fundamental one, 

 which bridges the incipient stages of characters 

 until natural selection can get a foothold, and 

 may even sweep onwards and completely 



K.G. 3. — Wing of Colitmha ae-ias, related to C. Iwia, Tlie 

 tvv7 bars are incmplete. 



erase a pattern which selection would have 

 retained. 



That something other than selection is at work 

 on these patterns is indicated also in such species 

 as the European stock-dove, Colnntha aetias 

 (Fig'- 3)- 'f 'his species the reduction of the bars 

 has proceeded farther than in the rock pigeon, 

 but the spots composing some of the partial bars 

 are completely concealed by oxerlying feathers. 



The same process is studied in great detail in 

 the crested pigeons of Australia, the phylogeny 

 being interpreted as' "always advancing in one 

 predetermined direction, like a tidal flow guided 

 along a prepared channel, and flowing to varying 

 distances, according to the initial momentum." 



(3) Perfect continuity in development and varia- 

 tion is another feature which Whitman is at great 

 pains to demonstrate. By plucking feathers at 

 intervals from the juvenal plumage, he showed 

 that imderlying the apparent discontinuity in 

 pattern between one moult and the next there was 

 complete continuity of the underlying physiological 

 processes. The exact nature of this physiological 

 developmental continuity is a nice problem on 

 which we have very little light at the present 

 time. But the author carries his conception of 

 continuity much farther. In his many crossing 

 NO, 2622, VOL. 104] 



experiments he found usually a blending result, 

 with a fluid condition and no halting places from 

 character to character. Granting that this may 

 be generally true of species cros.ses in pigeons, 

 yet the author himself describes cases of sex- 

 linked inheritance and other phenomena which 

 indicate some fixed boundary line between certain 

 characters. We therefore feel that a universal 

 philosophy of continuity is misleading because 

 untrue. The enormous Mendelian literature is not 

 based entirely upon superficial or hasty observa- 

 tion, although such cases no doubt occur. The 

 author would have been on safer ground had he 

 recognised, with Galton, that both continuity and 

 discontinuity exist in Nature, and both are equally 

 worthy of an explanation. We may perhaps look 

 forward to a harmonisation of these two opposing 

 principles on the basis of cell structure. 



(4) Several aberrant birds arising in these ex- 

 periments are described as "mutations." For 

 example, a Zenaida (Fig. 4, a) produced a mutant 

 in which the juvenal plumage (Fig. 4, h) was more 

 primitive than in the type, but the adult plumage 



m^ ^ ^ 



(') 



(«) 



Fig. 4. — ('z) Zenaida vifiac o-rii/a, normal juvenal plumage. The 

 light apical edge of the fe.ithers is a primitive character in birds. 

 {h) Mutant, juvenal plumage, showing a mesial extension of the 

 light apical tdge Thi.s condition is a -specific character in 

 C. gnittea, 



was normal. This condition was transmitted 

 through five generations in a generic cross. It 

 recalls a type of yellow seedling occurring in 

 maize which, if carefully nurtured, finally 

 grows up into a green plant. Similarly, a 

 Japanese turtle-dove (Fig. 5) produced three 

 partial albinos in her old age. In this case there 

 was also inbreeding. The orthogenetic interpreta- 

 tion assumes that the colour pattern is being pro- 

 gressively reduced, and that albinism is the final 

 condition to which the whole group of pigeons is 

 tending. This being the case, we may expect the 

 reduction series to be hastened in a weakened 

 germ, so that a long step in this direction might 

 be anticipated. Here it would seem possible to 

 find a basis for adjusting the conception of muta- 

 tion ■ft'ith that of orthogenesis, even in pigeons, 

 (s) The conception of germinal weakness or 

 I strength is one on which Whitman lays great 

 stress, and it is supported by much experimental 

 evidence. Differences in " strength " or develop- 



