56 THE INFLUENCE OF INANIBIATE SUEKOUNDINGS. 



the mouth ; but it is an exception which, as we see, is due to 

 the propensity of the species of Dasijpeltis to make the eggs of 

 birds their exclusive food. 



These few examples, to which others could easily be added, 

 for many are universally known, will here siifEce to prove 

 that monophagy in animals is often connected with the 

 occurrence of special organs or relations of structure, and that 

 the preservation of such species is solely due to their efficiency. 

 Their inefficient development would infallibly lead to the 

 destruction of the species — taking it for granted, of course, 

 that it was unable to accustom itself to any other food. 



Sometimes adaptation to a single kind of nourishment does 

 not depend, as in the cases here considered, on the existence of 

 a special organ, but on a peculiar cycle of development in each 

 individual axumal. This, for instance, is the case with all the 

 Intestinal Worms. These must become extinct if their larvse 

 were not able, or even forced, to migrate and to seek food in 

 other spots away from their parents. If we suppose that the 

 Tapeworm, or even the Trichina, were capable of going through 

 the whole cycle of its development within the same host, its 

 permanence as a species would be possible only if all men were 

 habitually cannibals. Corresponding to this we find that all 

 Intestinal Worms have to go through a longer or shorter 

 period of migratory existence as young and sexless creatures 

 or as larvse. They at the same time change their host several 

 times — for they often become parasites from the first, after a 

 short period of free life in the water — till at length they are 

 sexually mature, and have found their way into an animal or 

 an organ similar to that which they left in the embryo or larva 

 state. All internal parasites are subject to this inevitable law 

 of migration — such, that is to say, as live in the interior of an 

 animal structure or of its organs. It is applicable even to the 

 well-known Trichina spiralis, which is capable of going through 

 all the stages of its development in the same animal, but which 

 nevertheless travels, in its youth, from the intestine outwards to 

 the muscles. From them, however, it is incapable of returning 

 to the same intestine, although it would be perfectly capable of 

 achieving sexual development there and of producing eggs ; it 



