THE FACTORS OF EVOLUTION 155 



the mollusc into a so-called sporocyst. By budding, the sporo- 

 cyst produces a second larval form, the redia, and this again 

 a third, the cercaria, a little trematode furnished with a strong 

 paddle-tail. 



The task now before the cercaria is to pass from the snail 

 into the open. Having succeeded in this, it swims about in the 

 water searching for a new intermediate host, adopting as such 

 usually small crustaceans or insect larvae. If chance should 

 carry the cercaria into the body of a suitable animal it throws 

 off the tail which has now become useless, surrounds itself with 

 a firm shell, and changes in the cyst into the young, but as yet 

 sexless, liver-fluke. Finally, it is necessary that the second inter- 

 mediate host containing the parasite is eaten by the final host, 

 a vertebrate. This happens if sheep and cattle graze on wet 

 meadows. The arthropod is digested by the juices of the 

 stomach, the cyst is dissolved, and the young liver-fluke is 

 liberated from its prison.' All that now remains to complete its 

 metamorphosis into a mature trematode is to invade the bile- 

 ducts : with that it has reached the end of its journey and can 

 proceed to the reproduction of its species. We can now readily 

 understand that the fertility of species must stand in strict pro- 

 portion to the dangers to which the development of its individuals 

 is exposed. 



In addition to the vicissitudes of the life-cycle, the question of 

 food and of natural enemies play an important part. For instance, 

 the butterflies and caterpillars of a district are chiefly kept down 

 by the number of insectivorous birds, in particular the tits, by 

 ichneumon flies, and numerous other parasites which prey upon 

 them and their brood. A severe winter and want of food, causing 

 destruction among their natural enemies, is usually followed by 

 an enormous increase in the caterpillar pest. Thus a plague of 

 one of the most noxious insects of our forests, the ' nun ' 

 (Psilura monacha), is usually due to a previous reduction in the 

 number of its natural enemies. What enormous dimensions this 

 pest may under favourable conditions assume was shown in 

 1890-1891 when, in the kingdom of Bavaria alone, about 12,000 

 hectares of State forests were destroyed, and over 125,000 

 spent on exterminating this pest. Fortunately, an increase in 

 the pest of caterpillars always brings about, by the super- 



