276 DARWINISM AND THE PROBLEMS OF LIFE 



the fact that the stages of the parasites in the first hosts 

 have some resemblance to their primitive forms, from 

 which we may infer that these stages were at first 

 sexually mature animals. 



However, we will not linger over these theories, 

 but pass on to study the life-story of some of the 

 chief parasites. In this we shall see what a wonderful 

 adaptiveness there is in these creatures. 



One of the worst human parasites is the trichina. 

 The man who is visited by this dire guest has often to 

 pay for it with his life. It is chiefly on its account that 

 the examination of meat has been introduced, and 

 this and the now prevalent custom of killing in 

 abattoirs have greatly diminished the danger from 

 trichina. 



The trichina is found in a large number of animals, 

 but only one of these, the pig, calls for our notice. It 

 often lives in great quantities in the flesh of pigs, in the 

 shape of a small white point ; this is a capsule, and the 

 tiny worm is rolled up spirally inside it. If the pig's 

 flesh containing the capsules enters a man's stomach, 

 the capsules burst and the little worms issue forth ; they 

 travel into the small intestine and there reach sexual 

 maturity within a few days. The females give birth 

 to an enormous number of young, and then die. The 

 young pass through the wall of the intestine, which they 

 can easily do on account of their small size and pointed 

 shape, and travel gradually by the blood-vessels into the 

 man's muscles. There they feed for a time on the 

 decaying muscular matter, until this in turn secretes a 

 membrane to protect itself against the parasites. This 



