296 THE EFFECT OF FOOD 



fed both flesh-feeding birds on grain, and grain-feeders 

 on flesh, states that he was unable to trace any adapta- 

 tion to the altered conditions in either case. 



The Effects of Products of Metabolism. That organ- 

 isms react on each othe*r has long been recognised. The 

 interdependence is especially obvious in the case of 

 parasite and host ; but reflection will show, I think, that 

 the interaction is of much wider scope than is included 

 in such self-evident cases as these. In any given 

 volume of water, or any given area of land, every ani- 

 mal and every vegetable organism may to some extent 

 affect the well-being of every other organism, both ani- 

 mal and vegetable. The animal does this largely 

 through the agency of its own specific metabolism, or 

 through the specific products of excretion which, com- 

 ing into contact with the other organisms, in turn affect 

 them. That every species of animal does possess a 

 specific metabolism is, perhaps, scarcely what one would 

 on a priori grounds expect; but the observations made 

 by the author * tend to prove that such is actually the 

 case. These observations chiefly concern Echinoids, 

 both adult forms and plutei, but more especially the al- 

 ready so frequently mentioned plutei of Strongylo- 

 centrotus. 



On allowing the fertilised ova of Strongylocentrotus 

 or of Echinus microtuberculatus to develop in water in 

 which another batcfi of larvae (Strongylocentrotus, 

 Spharechinus or Echinus) had already been developing 

 for 8 to 12 days, but from which they had been re- 

 moved by filtration, it was found that in every case they 



* Vide Mittheilungen a. d. Zool. Stat. z. Neapel., Bd. xiii. p. 389 

 et seq. 



