THE REPRODUCTIVE FACTOR IN EVOLUTION. 



2S5 



probability, may of course have rapidly justified their existence in 

 the struggle for existence, just as unions ol many kinds do in human 

 society, but the Protozoa can not be accused of any prevision of 

 future advantage in remaining clubbed together in cooperation, nor 

 indeed credited with much primitive altruism in so doing. None the 

 less is it clear that this greatest of morphological steps was directly 

 due, not to any struggle, but rather to an organic sociality, or at any 

 rate to a process which is not interpretable in terms of individual . 

 advantage. 



No structure is more emphatically nutritive in its adult results than 

 the gut-cavity of the embryonic gastrula. It is worth inquiring 



~i (*) 



Fig. T03. — Formation of the Gastrula. — From Haeclrel. 



whether this important step in differentiation was attained in history 

 in response to nutritive needs. The usual supposition is certainly 

 that the gastrula cavity, by whatever peculiarities of growth it may 

 have arisen, justified itself from the first in an additional nutritive 

 advantage. But Salensky, in his studies on the primitive form of the 

 Mctazoa, has given strong arguments in favor of the theory that the 

 primitive cavity, arising in a volvox-like form, was originally a brood- 

 cavity or " genitoccel," and that it only secondarily acquired nutritive 

 significance. It would be indeed striking if this important morpho- 

 logical step in the establishment of the nutritive system was reached 

 along the road of reproductive modification; for if this most funda- 

 mental of nutritive and self-maintaining advantages, the belly itself 



I 



