EECIPLOCAL IHFLUEKCES. 331 



necessarily be sensible of the changes and possibly peiish in con- 

 sequence. Again, if certain insects whose function it is to 

 fertilise certain plants die out, others living as parasites on 

 those plants would necessarily suffer in a variety of ways. A 

 few species, being wholly monophagous, would perish com- 

 pletely ; polyphagous species, as it would seem, might be able 

 to adapt themselves, without suffering in the least, to a changed 

 form of nourishment and mode of life ; others again, if they were 

 not extirpated, would probably find themselves affected ia some 

 way or other by the change of food, and the effects might very 

 likely exhibit themselves by a certain modification iu the struc- 

 ture of their organs. We have mentioned some cases of this 

 kind in the chapter on Food ; it will therefore be superfluous to 

 dwell on them here, and all the more so as I have dealt with 

 food as one of the inanimate conditions of existence. 



But the reciprocal action of living organisms gives rise to 

 yet other modifications which can be in no way connected with 

 those occasioned by nutrition, even though this may be derived 

 from the organic kingdom. Thus, for instance, there are highly 

 intimate relations between the different individuals of the same 

 species and between the two sexes of a species ; many animals 

 are directly dependent upon others, although not using them in 

 any way as food ; to these belong the commensals or messmates, 

 which do not feed on their host or on its organs, although they 

 make use of them as a means of procuring the food that suits 

 them. 



As a rule the species thus thrown into juxtaposition are 

 reciprocally dependent on each other. If a parasite only 

 destroys the internal germ-gland of its host — as is not unfre- 

 quently the case with parasitic worms and crustaceans — the 

 host wiU not be rendered incapable of living, but only of propa- 

 gating its species. Thus I have never yet heard of the hermit- 

 crab, Pagurus, being found infested with parasitic Ciixhipedia of 

 the genus Peltogaster and at the same time carrying eggs on its 

 hind feet; nevertheless, the crabs continue to grow, and are to all 

 appearance healthy. In a similar manner the peculiar larvae of 

 certain Trematode worms eat away the reproductive glands of 

 pond-snails so completely that they are incapable either of fer- 



