u6 RETROGRESSION 



domestic bird, and so permits the reappearance of the ancestral 

 traits, may present every appearance of progression. Obviously, 

 the domestic characters which were new to the race and which 

 rendered latent more ancient traits, result from progression, and 

 their disappearance and the consequent reappearance of the latent 

 wild characters is an act of retrogression as well as reversion. It 

 may happen, of course, as normally occurs in the case of the 

 sexual characters, that the traits of the domestic fowls are not 

 eliminated when the wild plumage reappears, but replace the latter 

 as dormant characters, and therefore that they may reappear in 

 descendants, and even alternate with the wild traits as the sexual 

 characters alternate with one another ; but the fact that it is 

 sometimes extremely difficult to breed out hitherto latent ancestral 

 characters when once they have reappeared, seems to indicate that 

 their recrudescence may occasionally be due to a loss of the newer 

 domestic characters, not merely to the latency of the latter. 1 



190. It will be worth the reader's time to try to imagine an act 

 of retrogression which is not also an act of reversion. To take an 

 extreme example : suppose a child is born lacking a limb. Experi- 

 ence shows that a deficiency so great occurring in a single genera- 

 tion usually indicates latency rather than loss. But suppose it is an 

 instance of real retrogression i.e. of the total and permanent loss 

 of a complex hereditary tendency from the germ-plasm then 

 clearly the child has reverted to that enormously remote ancestor 

 in whom the limb did not exist. It is not maintained of course 

 that there ever was a period during the life-history when three 

 perfect limbs were possessed by the race while the fourth was 

 lacking. But since every part is independently variable, it is 

 necessary in this connection to think of every part separately. 



191. Not only is the limb independently variable, but each 

 smallest part of it possesses the same power. Suppose a child 

 comes into existence with a malformed limb. This variation may 

 be compounded of any number of smaller variations, some of which 



1 " I may here add a remark made by Mr Wicking, who has had more experi- 

 ence than any other person in England in breeding pigeons of various colours, 

 namely, that when a blue, or a chequered bird, having black wing bars, once 

 appears in any race and is allowed to breed, these characters are so strongly 

 transmitted that it is extremely difficult to eradicate them " (Animals and Plants, 

 vol. i. p. 210). On the other hand if, for example, albino mice be crossed with 

 black-and-white Japanese waltzing mice, the ancestral grey colour appears, but 

 if the mongrels be inter-bred, the albino and the black-and-white types reappear. 

 In this case the albino and the black-and-white colour become merely dormant 

 when the grey reappears. Medelians will dispute the inference, but it can be 

 substantiated (see chapter vii.). 



