IV,] FOUNDATION OF A THEORY OF HEREDITY. 1 85 



as many different effects as could have been produced by 

 differences of constitution. If an}^ one were to assert that in 

 Daphnidae^ or in any other forms which produce two kinds of 

 eggs, the power of developing only after a period of rest, 

 possessed b}'^ the winter-eggs, is based upon the fact that their 

 idioplasm is identical with that of the summer-eggs, but is in 

 another condition of tension, I should think such a hypothesis 

 would be well worth consideration, for the animals which arise 

 from the winter-eggs are identical with those produced in 

 summer : the idioplasm which caused their formation must 

 therefore be identical in its constitution ; and can only differ in 

 the two cases, as water differs from ice. But the case is quite 

 otherwise in the stages of ontogeny. How many different 

 conditions of tension ought to be possessed by one and the 

 same idioplasm in order to correspond to the thousand different 

 structures and differentiations of cells in one of the higher 

 organisms ? In fact it would be hardly possible to form even 

 an approximate conception of an explanation based upon mere 

 ' conditions of tensions and movement.' But, furthermore, 

 difference in effect should correspond, at any rate to some 

 extent, with difference in cause : thus the idioplasm of a muscle- 

 cell ought to differ more from that of a nerve-cell and of a 

 digestive-cell in the same individual, than the idioplasm of the 

 germ-cell of one individual differs from that of other individuals 

 of the same species ; and yet, according to Nageli, the latter 

 small difference in the effect is supposed to be due to difference 

 of quality in the cause — the idioplasm, while the former funda- 

 mental difference in the histological differentiation of cells 

 is supposed to follow from mere difference ' of tension and 

 movement.' 



Nageli's hypothesis appears to be self-contradictory ; for, 

 although its author recognizes the truth of the fundamental law 

 of development, and explains the stages of ontogeny as an 

 abbreviated recapitulation of phyletic stages, he nevertheless 

 explains the latter by a different principle from that which he 

 employs to explain the former. According to Nageli, the 

 stages of phylogcny are based upon true qualitative differences 

 in the idioplasm : the germ-plasm of a worm is qualitatively 

 different from that oi Amphioxiis, a frog, or a mammal. But if 

 such phyletic stages occur crowded together in the ontogeny of 



