ON SOME INTERACTIONS OF ORGANISMS." 
By S. A. FORBES. 
While the structural relations of living organisms, as 
expressed 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 all express their functional relations. W 7 hile 
the anatomical characters of the various groups may 
show that they are all branches of a common stock, from 
which they have arisen by repeated divisions and contin- 
ued divergencies, the history of their lives will show that 
they are now much more intimately and variously bound 
together by mutual interactions than are twigs of the 
same branch — that with respect to their vital activities 
they occupy rather the relation of organs of the same ani- 
mal body. If for a type of their classification we look to 
the vegetable world, for an illustration of their mutual 
actions and reactions we must look to the animal world. 
The serious modification of any group, either in num- 
bers, 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-extend- 
ing 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. 
