CHAP. XII. EVOLUTION IN CRUSTACEA. 133 



common stem, a simplification of their process of deve- 

 lopment took place. This also seems to be intelligible 

 from Darwin's theory. When any circumstances favour- 

 able to a group of animals caused its wider diffusion 

 and divergence into forms adapting themselves to new 

 and various conditions of existence, this greater varia- 

 bility, which betrays itself in the production of new 

 forms, will also favour the simplification of the deve- 

 lopment which is almost always advantageous, and 

 moreover, exactly at this period, during adaptation to 

 new circumstances, as has already been indicated with . 

 regard to fresh-water animals, this simplification will 

 be doubly beneficial, and therefore, in connexion with 

 this, a doubly strict selection will take place. 



So much for the development of the higher Crustacea. 



A closer examination of the developmental history 

 of the lower Crustacea is unnecessary after what has 

 been said in general upon the historical significance of 

 the young states, and the application of this which has 

 just been made to the Malacostraca. We may see, 

 without further discussion, how the representation given 

 by Claus of the development of the Copepoda may pass 

 almost word for word as the primitive history of those 

 animals ; we may find in the Nauplius-skin of the larvae 

 of Achtheres and in the egg-like larva of Cryptophialus, 

 precisely similar traces of a transition towards direct 

 development, as were presented by the Nauplius-euve- 

 lope of the embryos of Mysis and the maggot-like larva 

 of Ligia, &c. 



It will be sufficient to indicate an essential difference 



