188 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 
