320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



is evidently a natural law that no food supply is left untouched. 

 Further, the natural law seems to decree that although all food 

 supplies may be utilized, none may be utterly consumed. 



It is a general habit of animals to sample foods here and there; 

 rarely do they make a clean sweep of anything. This habit con- 

 tributes to biologic balance, as the toll taken is not so great but 

 that the remainder is sufficient to maintain the food species in about 

 their average abundance. Thus the greenery about us looks much 

 the same from year to year; the insects dependent on that foliage 

 neither increase nor decrease except sporadically; and birds that 

 prey upon the insects maintain their average numbers. 



These things speak eloquently of biologic balance, and there is a 

 reason. Nature, while tolerant and slow, is inexorable. If a species 

 too largely consumes its food supply, its own numbers will decrease. 

 It may live comfortably on "interest" for years, but let it eat into the 

 "principal" and its own account in the bank of life will soon be 

 overdrawn. 



The workings of nature's balance are evident not merely in a 

 broad sense but also locally. In fact natural law is the summation 

 of local happenings. Living things, as a rule, are very localized; 

 as to plants the condition is obvious, but it is likewise true that 

 individual animals do not range widely. Migrants are an apparent, 

 rather than a real, exception, for individuals and groups keep 

 to certain areas in both summer and winter homes ; even their migra- 

 tion routes are relatively fixed. In general, territorialism rules, and 

 it contributes a great deal toward balance. 



Territorialism is the name we have for nature's system of parceling 

 out places to live. An individual plant occupies a comparatively small 

 territory — that traversed by its roots and branches. Usually, condi- 

 tions are not so uniform but that some other plant is a little better fitted 

 to occupy an adjacent nook, whether made by vertical or horizontal 

 variations in soil or moisture. Mixtures are the rule, pure stands the 

 exception. Each plant draws certain substances from the soil and adds 

 others to it, thus maintaining average fertility. To resume the banking 

 metaphor, demands and repayments by each living thing are different, 

 but in the long run a fair balance is struck. 



In contrast to plants, animals seem very free-moving, but even their 

 movements are limited. The territory of a pair of small birds in the 

 breeding season may be less than an acre in extent, and a family of 

 bobwhites may never range more than a quarter of a mile. Mice may 

 be restricted to a fraction of an acre, squirrels to a radius of a few hun- 

 dred yards, and cottontails to from one to several acres. In general, 

 the larger the animal the more extensive the territory ; but in no event 

 is the individual range indefinite. 



