MIGEATION. 57 



must absolutely pass out from the muscles to the intestine of 

 some other creature — a rat, a mouse, &c. — in order to pass, in 

 the second generation, back again into the human intestine. If 

 the Trichina from the muscle does not pass again into the 

 intestine of some other creature — which is, of course, commonly 

 the case as regai-ds human beings — it infallibly dies, although its 

 tenacity of life is enormous ; such a Trichina can live for ten 

 years enclosed in a muscle. Here the permanence of the species, 

 as such, depends on the capability of the larvae for migration, 

 and for finding their nutriment in other animals which may 

 secure their transfer into those in which alone they can find 

 the special food that is necessary for their full development, 

 and for the exercise of their sexual functions. If a young para- 

 site were to lose its way, or to be swallowed by an unsuitable 

 host— a Trichina, for instance, by a Fish — it would infallibly 

 perish unless it were able to accustom itself quickly to the 

 food which is unsuited to it. No such cases, however, are 

 known of adaptation of parasitical worms, when sexually 

 mature, to an unwonted form of nutriment. Thus it would 

 appear as if in these cases, without exception, the change of food 

 involved in the migration of the young animal were of the 

 same service to the species as are special organs contrived for 

 special nutrition in others ; the preservation of the species in 

 the former class depends on change of food and migration 

 just as much as, in the latter, it depends on the adaptation and 

 functional activity of individual organs. 



A similar dependence of the species oai its food does not of 

 course exist among the truly Polyphagous Animals. Their 

 polyphagous habits allow of their changing their food at 

 pleasure without suffering in any way, or at any rate seriously, 

 when, from any external cause, they are obliged to alter their 

 mode of life. It must not however be forgotten that even these 

 animals depend to a certain, if not to a very great degree, on the 

 nature of their food. It is now universally admitted that in 

 many animals a definite relation must subsist between the 

 amount and kind of food if the animal is to derive the greatest 

 possible advantage fi'om the food consumed. Man in this 

 respect offers the best known instance. Starchy food or sugar, 



