1 38 MODERN IDEAS OF EVOLUTION 



the egg, which is potentially the hen, as of the hen 

 itself. Thus the similarity of the eggs and early em 

 bryos of animals of different grades is apparent only ; 

 and this fact, which embodies a great and perhaps in 

 soluble mystery, invalidates the whole of Haeckel s 

 reasoning on the alleged resemblances of different 

 kinds of animals in their early stages. 



A second difficulty arises from the fact that the 

 simple embryo-cell of any of the higher animals 

 rapidly produces various kinds of specialised cells, 

 different in structure and appearance and capable of 

 performing different functions, whereas in the lower 

 forms of life such cells may remain simple, or may 

 merely produce several similar cells little or not at all 

 differentiated. This objection, whenever it occurs, 

 Haeckel endeavours to turn by the assertion that a 

 complex animal is merely an aggregate of inde 

 pendent cells, each of which is a sort of individual. 

 He thus tries to break up the integrity of the com 

 plex organism and to reduce it to a mere swarm of 

 monads. He compares the cells of an organism to 

 the individuals of a savage community, who, at first 

 separate and all alike in their habits and occupations, 

 at length organise themselves into a community and 

 assume different avocations. Single cells, he says, at 

 first were alike, and each performed the same simple 

 offices as the others : At a later period isolated 

 cells gathered into communities, groups of simple 

 cells, which had arisen from the continued division of 



