1 88 THE DOCTRINE OF DESCENT. 



the alimentary canal has become entirely unnecessary; 

 but they exhibit neither intermediate forms nor phases 

 of development. It is different, however, with the para- 

 sitic Crustacean, for here the young, locomotive, and 

 well-integrated being as its protype in definitive ge- 

 neric forms permanently locomotive, from which, after 

 adhesion, it deteriorates into a mere motionless sac. All 

 these animals, including the intestinal worms, have ac- 

 quired their position and status (and this is the true 

 significance of parasitic life) by the apparent degrada- 

 tion of their organization. They are, almost without 

 exception, distinguished by their reproductive power; 

 and on this, owing to the easy supply of nutriment, 

 without any exertion of the other parts of the or- 

 ganic system, the whole bodily activity could be concen- 

 trated. 



We have hitherto demonstrated that organisms are 

 urged to continual differentiation by the unremitting 

 struggle for existence. For the cultivation of morpho- 

 logical species, natural selection, moreover, seizes on the 

 modifications arising from the mere variability of the 

 organism, and implying no physiological advance. But 

 sooner or later these are also inevitably drawn into the 

 vortex of fcompetition. After what has been already 

 said, this fact is so self-evident as to need no further 

 proof. Even did we not see the infinite variety of or- 

 ganisms, a divergence into novelty must needs be in- 

 ferred on a priori grounds, from the existence of the 

 simple and uniform, and the necessity of adaptation to 

 altered external conditions. But with development in 

 various directions, under the guidance of natural selec- 

 tion, progress is necessarily combined. It is one 



