164 



THE EVOLUTION THEOEY 



through this whole development within the egg, and emerges 

 perfectljr formed. 



We see from this example that it i^jio ^some inward necessit y 

 whichthus, 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 ada ptation, mainly to 



FiGi. 109. D, Mysis-stiige. Thirteen pjiirs of appendages are now formed : 

 / and II, antennse ; III, mandibles ; IV and Y, maxillae ; VI-XIII, swimming 

 appendages with one branch or with two. AM, abdonien. SJl, tail-fin. 

 E, the fully-formed Shi-imp, with thirteen pairs of appendages on the 

 cephalothorax (Cph) ; I and II, the two pairs of antennas ; then follow the 

 maxillaj and maxillipedes (IIl-VIII), the last of which is visible in the figure, 

 and the five pairs of walking- legs (JX-XJiZ) of which the third bears a long 

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



the_conditions_ofJaxYaLlif e. 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 species, 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 



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