INTEBEELATIOXS OF LIVING THINGS 37 



live amid the foliage of the tree. In all these cases the substance of the 

 tree is levied upon by the insects for food, and if levied upon unduly, the 

 trees suffer commensurately. But, as counteracting factors, we find corre- 

 sponding categories of birds, each specially equipped to make use of one 

 of these categories of insects. The woodpeckers, nuthatches, and creepers 

 search the tree trunks and larger limbs; the chickadees comb the finer 

 twigs ; while the kinglets and warblers go over the foliage leaf by leaf. The 

 great value of the bird to the tree comes when the harmful insects have 

 begun to multiply abnormally ; for birds are well known to turn from other 

 food sources and concentrate upon the one suddenly offering in generous 

 measure. 



It is to the interest of the forest at large that a reserve nucleus of birds 

 be maintained constantly, as a form of insurance, to be ready at just such 

 a critical time. Incursions of insects from neighboring areas, as well as 

 eruptions of endemic species, have probably occurred again and again from 

 remote times. In other words, as we see the situation, it is an advantage 

 to the forest that a continual moderate supply of insects be maintained 

 for the support of a standing army of insectivorous birds, which army will 

 turn its attention to whatever insect plague happens suddenly to manifest 

 itself. 



We would claim, then, a nice interdependence, an adjustment, by which 

 the insect and the bird, the bird and the tree, the tree and the insect, all 

 are, under average circumstances, mutually benefited. Such a balance is 

 to be found in the primeval forest, where thoroughly 'natural' conditions 

 obtain as a result of long ages of evolution on the part of all the animate 

 things there touching upon one another's lives. These relations may, 

 of course, be entirely upset where man has interfered, directly or indirectly ; 

 as, for instance, when he brings in insects or plants alien to the original 

 fauna and flora. Then an entirely new program, one of readjustment, 

 begins. 



After a good deal of study, and contemplation of the modes of life of 

 various kinds of animals, naturalists have come to recognize as essential 

 three factors which seem inseparably bound up with the successful existence 

 of any one species of vertebrate animal. These factors are: (1) presence 

 of safe breeding places, adapted to the varying needs of the animal; in 

 other words, depending upon the inherent powers of construction, defense, 

 and concealment in the species concerned, (2) Presence of places of 

 temporary refuge for individuals, during daytime or night-time, or while 

 foraging, when hard pressed by predatory enemies, again correlated with 

 the inherent powers of defense and concealment of the species involved. 

 (3) Kind of food supply afforded, with regard, of course, to the inherent 

 structural powers in the animal concerned to make it available. 



