302 THE WONDER OF LIFE 



worms and flukes are known to fertilize their own eggs. 

 External parasites are naturally much less degenerate than 

 internal parasites ; the retrogression is proportionate to 

 the thoroughness of the parasitism. 



It is characteristic of parasites to be prolific. Some of 

 the tapeworms are said to produce eight millions of eggs ; 

 the female Trichina gives birth viviparously to fifteen 

 hundred young ones ; a liver-fluke is said to produce some 

 fifty thousand eggs. There are two ways of looking at this 

 prolific productivity. On the one hand, as regards the 

 individual organism, it is living without much exertion, 

 with abundance of stimulating food at its disposal. It is 

 physiologically in a position to be prolific. On the other 

 hand, as regards the race, there can be no doubt that the 

 prolificness is adaptive, that is to say, those types of parasite 

 have survived which were constitutionally prolific. The 

 risks in the life of a parasite are very slight when it is en- 

 sconced within its host, but they are often enormous in the 

 juvenile stages, or when there is transference from one host 

 to another. The life-history of the liver-fluke and the ox- 

 warble, subsequently referred to, may be taken as good 

 instances of these risks, but they are very general. 

 It must not be supposed that the prolific reproduction was 

 evolved as a reponse to these great risks ; it is rather to be 

 believed that those parasites which were constitutionally 

 prolific have become the surviving parasites. There are 

 good reasons for supposing that the parasitic alternative 

 is always being attempted and has always been attempted, 

 but that many of those organisms admitted to the available 

 asylums have died out within them. The dog is known 

 to have about forty different parasites ; both man and 

 the pig have more. The Scoter duck (CEdemia nigra) 



