182 THE CONTINUITY OF THE GERM-PLASM AS THE 



weighty objections can be raised. At present I will only men- 

 tion that the meaning of the phrase 'conditions of tension and 

 movement ' ought to be made clear, and that we ought to be 

 informed how it is that mere differences in tension can produce 

 as many different effects as could have been produced by differences 

 of constitution. If any one were to assert that in Daphnidae, or in 

 any other forms which produce two kinds of eggs, the power of de- 

 veloping only after a period of rest, possessed by 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 organ- 

 isms? In fact it would be hardly possible to form even an 

 approximate conception of an explanation based upon mere ' con- 

 ditions 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 fundamental difference in the his- 

 tological differentiation of cells is supposed to follow from mere 

 difference ' of tension and movement.' 



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

 though its author recognizes the truth of the fundamental law of 

 development, and explains the stages of ontogeny as an abbre- 

 viated recapitulation of phyletic stages, he nevertheless explains 

 the latter by a different principle from that which he employs to 

 explain the former. According to Niigeli, the stages of phylogcny 

 are based upon true qualitative differences in the idioplasm : the 



