384 THE EVOLUTION THEORY 



suited. Thus the mammals are lung- breathers, and their extremities 

 are obviously adapted for locomotion on the solid earth, j^et several 

 grouj)S have returned to aquatic life, as, for instance, the family of 

 otters and the orders of seals and whales. Thus among insects which 

 are adapted for direct air-breathing, certain families and stages of 

 development have returned to aquatic life, and have developed 

 breathing- tubes by means of which they can suck in air from the 

 surface of the water into their tracheal system, or so-called tracheal 

 gills, into which the air from the water diffuses. But the most con- 

 vincing proof of the organism's power of adaptation is to be found 

 in the fact that the possibility of living parasitically within other 

 animals is taken advantage of in the fullest manner, and by the 

 most diverse groups, and that their bodies exhibit the most marvellous 

 and far-reaching adaptations to the special conditions prevailing 

 within the bodies of other animals. We have already referred to 

 the high degree reached by these adaptive changes, how the parasite 

 may depart entirely from the type of its family or order, so that 

 its relationship is difficult to recognize. Not only have numerous 

 species of flat worms and round worms done this, but we find 

 numerous parasites among the great class of Crustaceans ; there 

 are some among spiders, insects, medusoids, and snails, and there 

 are even isolated cases among fishes. 



If we consider the number of obstacles that have to be overcome 

 in existence within other animals, and how difficult and how much 

 a matter of chance it must be even to reach to such a place as, for 

 instance, the intestine, the liver, the lungs, or even the brain or the 

 blood of another animal, and when, on the other hand, we know how 

 exactly things are now regulated for every parasitic species so that 

 its existence is secured notwithstanding its dependence upon chance, 

 we must undoubtedly form a high estimate of the plasticity of the 

 forms of life and their adaptability. And this impression will only 

 be strengthened when we remember that the majority of internal 

 parasites do not pass directl}'' from one host to another, but do so only 

 through their descendants, and that these descendants, too, must 

 undergo the most far-reaching and often unexpected adaptations in 

 relation to their distribution, their penetration into a new host, and 

 their migrations and change of form within it, if the existence of the 

 species is to be secured. 



We are tempted to study these relations moi'e closely ; but it is 

 now time to sum up, and we must no longer lose ourselves in wealth 

 of detail. Moreover, the life-history of many parasites, and of the 

 tape- worm in particular, is widely known, and any one can easily fill 



