and large, over any long period, the average 

 level of the population remains fairly con- 

 stant, provided environmental conditions do 

 not undergo a permanent change. 



Limitations of the Environment. In general, 

 the principal factor that limits a given pop- 

 ulation is the food supply — using this term 

 in the very broadest sense. Among animals 

 and colorless plants, the fund of organic 

 foods available in the particular habitat is 

 especially important, although water, oxy- 

 gen, or other inorganic nutrients may be 

 limiting factors in some environments. But 

 among plants, the primary nutritive factor is 

 apt to be the quantity of accessible light, 

 although the available quantity of inorganic 

 foods, such as water or salts, is sometimes 

 very important. 



A number of other environmental factors 

 act directly in determining the population 

 levels of different animals and plants, in 

 addition to having indirect effects upon the 

 food supply. Climatic conditions (heat, cold, 

 wind, ice, snow, fog, etc.) impose definite 

 limits on the extent of territory that can be 

 occupied by a given species; predaceous and 

 parasitic enemies (Chap. 30) may prey upon 

 the populations, and a variety of minor fac- 

 tors may assume importance in particular 

 cases. But for any particular species, under 

 unchanging environmental conditions, a bal- 

 ance is reached between its natural tendency 

 to increase and the limiting factors of its 

 particular environment. 



NATURAL SELECTION 



The fact that each species in every genera- 

 tion produces so many offspring that only a 

 fraction can possibly achieve the age of 

 sexual maturity gives rise to two important 

 questions: What selecting agency scrutinizes 

 the many offspring and chooses the few that 

 are to survive and perpetuate the species; 

 and what characteristics qualify the indi- 

 viduals that are selected? 



Generally speaking, organisms that are 

 fittest to simjive and reproduce under the 



Natural Selection; Origin of Species - 541 



conditions of their particular environment 

 are automatically selected to perpetuate each 

 species. The selecting agency is the natural 

 environment of each species, and the main 

 qualification for selection is a fitness to sur- 

 vive and reproduce in competition with other 

 organisms that seek to utilize the same en- 

 vironment. Inevitably each organism en- 

 counters a ceaseless struggle for existence — 

 using this term in the broadest sense — since 

 potentially unlimited numbers of each kind 

 of organism are competing for sustenance in 

 a definitely limited environment. 



The struggle for existence does not imply 

 direct and active combat, except in the case 

 of some predaceous animals. Survival de- 

 pends more on the organisms' capacity to 

 withstand adverse physical conditions (tem- 

 perature, light, etc.) and unfavorable condi- 

 tions (drought, soil fertility, etc.) in the 

 chemical environment. But above all, the 

 struggle is a competition for the necessities 

 of life, of which the supply is strictly limited. 

 This aspect of the struggle for existence, 

 indirect and obscure though it frequently 

 may be, reaches a climax of intensity among 

 organisms of the same or closely related 

 species, because such organisms are compet- 

 ing for the same, or very similar necessities. 



Take, for example, a group of annual 

 plants sprouting on a limited plot of ground. 

 At first the ground may be thickly covered 

 with a large number of small seedlings; but 

 as the season progresses, the number of sur- 

 vivors falls off, while their size increases. 

 Finally there are just a few individuals left 

 and these completely occupy the area in 

 which hundreds of seeds originally germi- 

 nated. Among such plants the struggle is 

 primarily for a "place in the sun," and the 

 struggle is especially keen because light is 

 absolutely essential to every competitor. The 

 seedlings that spread their leaves a little 

 faster and farther overshadow the others, 

 cutting off their light and assuring their 

 starvation. But the survivors go on and pro- 

 duce the next year's crop of seeds, whereupon 

 the process is repeated. Thus a selection has 



