62 THE PRINCIPLES OF HEREDITY 



there is unlimited evidence of the existence of dormant 

 tendencies. " Besides the visible changes that it (the germ) 

 undergoes, we must believe that it is 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 female 

 ancestors separated by hundreds and even thousands of 

 generations irom the present time ; and these characters, 

 like those written on a paper with invisible ink, lie ready to 

 be evolved whenever the organism is disturbed by certain 

 known or unknown conditions." * 



103. Thus all domesticated races of pigeons have been 

 evolved under stringent artificial selection from the wild blue 

 rock. Every one of these varieties, no matter how purely 

 bred, occasionally produces an individual with the blue 

 ground-colour and the other special markings of the wild 

 ancestor. 2 Purely bred domesticated fowls show a similar 

 tendency by reverting in plumage to the wild Gallus lankiva? 

 Horses are sometimes faintly striped. Numerous similar 

 examples may be found among animals. Amongst many 

 varieties of cultivated plants reversion is more the rule than 

 the exception. The latency of certain inborn characters is 

 quite normal. The sexual characters are transmissible 

 through children of an opposite sex to grandchildren of the 

 same sex, and an old bantam hen, whose ovaria were diseased, 

 has been known not only to develop the warlike disposition 

 of a cock, but also to display in her tail the sickle-shaped 

 feathers of remote male ancestors. Haemophilia may be 

 transmitted through mothers who display no trace of it. 

 The tendency to gout and polydactylism may skip a genera- 

 tion. In cross-breeding the appearance of latent ancestral 



1 Animals and Plants, vol. ii., pp. 35-6. 



2 See Animals and Plants^ vol. i., p. 216. So far as the present 

 writer is aware no record exists of these reverted birds, when paired 

 together, reproducing the coloration of the domestic variety or varieties 

 from which they were derived. On the contrary, " When a blue, or a 

 blue and chequered bird, having black wing-bars, once reappears in any 

 race and is allowed to breed, these characters are so strongly transmitted 

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

 vol. i., p. 210.) In other words the white or black of the domesticated 

 variety is not rendered latent in the reverted descendant. If, however, 

 the hypothetical " ancestral units " concerning which so much has been 

 written had any real existence, the domesticated variety should reappear 

 almost infallibly, since the ancestors of the reverted bird must have been 

 red, white or black for, perhaps, hundreds of generations. Several 

 breeds of domesticated pigeons are known to have been in existence 

 three hundred years ago. Doubtless their real antiquity is much 

 greater. 



3 Animals and Plants, vol. ii., p. 251, 



