216 EXPERIMENTAL EVOLUTION LECT. 



not only changed in consequence of the immediate 

 effect of the surrounding elements, but also in con- 

 sequence of retarded development under their influ- 

 ence ; and, furthermore, that sexual maturity shows 

 itself in the salt-water generations earlier than the 

 complete typical development of the body-parts. 

 The termination of the sensory antennae, the colour 

 of the body, the lesser pinnulation of the bristles in 

 the salt-water generations are principally dependent 

 upon the immediate effect of the surrounding elements. 

 The smaller number of the above-mentioned spines 

 on the post-abdomen principally depends upon the 

 retarded development under the influence of changed 

 surroundings." 



We thus see that the change of environment makes 

 itself felt in various changes in the anatomy and 

 physiology of the species investigated. The same 

 conclusion holds good in the case of Branchipus 

 ferox, another Crustacean which inhabits both salt 

 and fresh waters. The differences relate to the 

 length which the egg-sac attains, the segments 

 of the animal, its shape, the length of the furcal 

 lobes, and the bristles of the latter. " The most 

 important difference," says Schmankewitsch, " con- 

 sists in that while in Branchipus ferox of our salt 

 ditches the furcal lobes have both edges bristled, in 

 the fresh-water form only the inner edges of the lobes 



