ON SOME INTERACTIONS OF ORGANISMS.* 



By S. a. FORBES. 



While the structural relations of living organisms, as ex- 

 pressed in a classification, can best be figured by a tree, — 

 the various groups, past and present, being related to each 

 other either as twigs to twigs, as twigs to branches, or as 

 branches to the main stem, — yet this illustration does not at 

 aW ex-press their functional relations. While the anatomical 

 characters of the various groups msiy show that they are all 

 branches of a common stock, from which they have arisen 

 by repeated divisions and continued divergencies, the his- 

 tory of their lives will show that they are now much more 

 intimately and variously bound together by mutual inter- 

 actions than are twigs of the same branch, — that with re- 

 spect to their vital activities they occupy rather the relation 

 of organs of the same animal body. If for a type of their clas- 

 sification we look to the vegetable world, for an illustra- 

 tion of their mutual actions and reactions we must look to 

 the animal world. The serious modification of any group, 

 either in numbers, habits, or distribution, must modify, 

 considerably, various other groups ; and each of these must 

 transmit the change in turn, or initiate some other form 

 of change, the disturbance thus propagating itself in a far- 

 extending circle. 



While the whole organic world, viewed as a living unit, 

 thus differs from the single plant by the much greater in- 



*As details accumulated relating to the food of animals and similar 

 subjects, it was found that a proper discussion of them would necessarily- 

 lead, step by step, to a full review of certain parts of the general subject 

 of the reactions between groups of organisms and their surroundings, 

 organic and inorganic. Without such a review, the facts can not be safely 

 generalized, nor the conclusions clearly apprehended to which they 

 point. It has therefore seemed best to prepare the way for the discussion 

 of special subjects by this general discussion of the subject at large. 



The practical importance of this larger view is illustrated by the fact 

 that if the current ideas of the value of parasitic and predaceous insects 

 are accepted, we must condemn the bluebird to extermination as a pest ; 

 while if the conclusions of this paper are essentially sound, this bird is 

 a very useful species and should be carefully preserved. 



