THE DARWINIAN THEORY 549 



crease that approximate the theoretical possibilities are seen in the 

 " plagues " of insects and other small animals that occur in years 

 when the conditions are favorable and an unusual percentage of 

 the eggs develop. We may, therefore, conclude that the possi- 

 bilities of great increase are always present, although never fully 

 realized. 



Total Numbers Stationary. — Despite the potentialities of 

 increase, as stated in the foregoing paragraphs, it is evident that 

 the total number of any species in nature must remain about sta- 

 tionary save for the annual fluctuations due to varying conditions 

 and the occasional expansions that occur when new territory is 

 invaded. Only a certain number of trees can find standing room 

 in a forest; only so many fish can obtain food in a stream; and 

 the number of insects and large herbivorous animals is determined 

 by the supph^ of food and by the number of enemies. Thus, 

 numbers are held stationary by what Darwin called the checks 

 upon increase. These are often very complex, involving inter- 

 relationships between different species, as in the case of mice and 

 clover (cf. p. 34). As Darwin says, " We know not exactly 

 what the checks are, even in a single instance." Food and stand- 

 ing room are the ultimate checks, but in most cases it is probable 

 that others intervene before these become operative. There is usu- 

 ally plenty of food for herbivorous animals, for their numbers are 

 held well below the food limits by carnivorous enemies, as where 

 rabbits are kept down by coyotes. Parasites play a role similar 

 to that of enemies. The young of plants and animals are destroyed 

 in great numbers; many seedling trees in a forest fail to find a 

 foothold, and countless young birds fall an easy prey to enemies 

 just after leaving the nest. Again, climatic and seasonal condi- 

 tions may act as checks. So effective are these checks that the 

 abundance of adult individuals in any species is not so much a 

 matter of the eggs or young produced as of the chances of destruc- 

 tion. Darwin says, '' The condor lays a couple of eggs, the ostrich 

 a score, and yet in the same country the condor may be the more 

 numerous of the two; the fulmar petrel lays but one egg, yet it is 

 believed to be the most numerous bird in the world." Again, 

 " one fly deposits hundreds of eggs, and another, like the hippo- 

 bosca, a single one; but this difference does not determine how 

 many individuals of the two species can be supported in a dis- 

 trict. It would suffice to keep up the full numbers of a tree, 



