114 



ancestor occasionally appear among domesticated pigeons. 

 These individuals continue to produce offspring with the 

 markings of the wild bird. " When a blue or a blue and 

 checkered 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." 1 



If a Japanese waltzing mouse is crossed with an albino, 

 the offspring are coloured in just the . same way as the wild 

 house-mouse. 2 Not only this, but although the ancestors 

 have been bred in captivity for many hundred generations 

 and have been tame, the offspring of this cross are just as 

 wild and timid as the wild mouse taken direct from its 

 natural conditions. The same tendency to revert to ancestors 

 is found in a great many other animals, and is extremely 

 common among plants. 



Most of the theories of inheritance explain these reappear- 

 ances of ancestral characters by assuming the existence of 

 individual entities representing them. 3 These entities are 

 supposed to be passed on from generation to generation, 

 some of them remaining dormant. A change of some kind 

 in the environment or in the egg itself stimulates the 

 dormant entity, and the ancient character it represents 

 reappears after having been in abeyance for perhaps many 

 hundreds of generations. The theory of recapitulation, on 

 the other hand, assumes no entities representing characters. 

 It assumes no more than was assumed for Nageli's idio- 

 plasm. 4 The substance of the fertilised ovum is such that 

 it is capable of developing along particular lines only, and 

 of producing only certain characters. We know that the 

 development of the offspring is a repetition of the develop- 

 ment of the parents, so that this theory really assumes no- 

 thing that cannot be demonstrated in the case of any normal 



1 Darwin, Animals and Plants under Domestication, vol. i. p. 210. 



2 Darbishire, Bivmetrica, vol. ii. pp. 101-165 and 282, 1902. 



3 Darwin, gemmules; Haeckel, plastidules ; Weismann, biophors ; Nageli 

 micella ; Galton, stirps ; de Vries, pangens. 



4 See p. 9. 



