164 



THE EVOLUTION THEORY 



through this whole development within the egg, and emerges 

 perfectly formed. 



We see from this example that it is not some inward necessity 

 which thus, in the higher and more complicated organism, contracts 

 the ontogeny into the embryonic state, but that this depends upon 

 external adaptive factors. Here again we have adaptation, mainly to 



Fig. 109. D, Mysis-stage. Thirteen p:\ir.s of appendages are now foi'ined : 

 / and II, antennee ; III, mandibles ; IV and V, maxillae ; VI-XIII, swimming 

 appendages with one branch or with two. Ahd, abdomen. SJl, tail-fin. 

 E, the fully-formed Shrimp, with thirteen pairs of aj^pendages on tlie 

 cephalothorax {Cpli); I and II, the two pairs of antennae; then follow the 

 maxillae and maxillipedes (III-VIII), the last of which is visible in the figure, 

 and the five pairs of walking-legs (IX-XIII) of which the third bears a long 

 chela. On the abdomen there are now six pairs of appendages (J^IV-XIJ^). 



the conditions of larval life. The elimination of the larvae by enemies, 

 for instance, will, other things being equal, be so much the more 

 incisive the longer the larval development is protracted, but in that 

 case the general ratio of elimination of the sj^ecies, and the degree of 

 fertility the species must possess in order to hold its own in the 

 struggle for existence, will also play a part in determining the mode 



