1 88 THE DOCTRINE OF DESCENT. 



particular organs on the configuration of the various 

 parasites. The parasitic Crustacea are perhaps the most 

 instructive, as they present the most complete systematic 

 series, exhibiting the gradual atrophy of the organs 

 which accompanies the ever-increasing connection of 

 the parasite with his host. In several orders of intes- 

 tinal worms, the alimentary canal has become entirely 

 unnecessary ; but they exhibit neither intermediate 

 forms nor phases of development It is different, 

 however, with the parasitic Crustacean, for here the 

 young, locomotive, and well-integrated being has 

 its prototype in definitive generic forms permanently 

 locomotive, from which, after adhesion, it deteriorates 

 into a mere motionless sac. All these animals, in- 

 cluding the intestinal worms, have acquired their 

 position and status (and this is the true significance of 

 parasitic life) by the apparent degradation of their 

 organization. They are, almost without exception, dis- 

 tinguished by their reproductive power; and on this, 

 owing to the easy supply of nutriment, without any 

 exertion of the other parts of the organic system, the 

 whole bodily activity could be concentrated. 



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 competition. 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 



