NATURAL SELECTION 389 



supply. Grass is a fine example of maximum crowding, but where 

 the periwinkle vine grows, it crowds out even grass, and where 

 the rainfall is slight, only sparse grasses can grow, since they nmst 

 draw water from a greater area of soil in order to live. 



The struggle between organisms may thus be passive competi- 

 tion for the same things, or open conflict. Between animals it 

 may be either, between plants it is the former. Animals may 

 fight for the right to a favorable range, and the conflict between 

 carnivorous species and their prey can only be direct competition. 

 In many cases, however, competition implies merely that different 

 organisms seek the same thing. If the supply is ample, both 

 succeed; if insufficient, then first come, first served, and the loser 

 must die or change his mode of life. In all cases the relationship 

 is essentially the same. The organisms concerned are actually 

 pitted against each other, and whether the conflict is of tooth and 

 claw, or merely a matter of the early bird's getting the worm, 

 some individuals must lose out. 



The Role of Useful Characters in Competition. It is in such 

 competition that the factor of usefulness is preeminent. Every 

 contact with the environment demands some power of response. 

 If struggle is merely in the form of passive competition for a 

 limited food supply, keenness of the senses involved in the 

 search for food may decide the winner. Some slight variation in 

 speed might well favor the faster animal in seeking the same 

 prey. Broad leaves might enable a plant in crowded ground to 

 secure necessary sunlight and at the same time deprive the small- 

 leaved neighbor in its shade of an adequate supply. The fact 

 that some plants vary to such an extent that they can meet the 

 requirements of very different environments emphasizes the 

 importance of such conditions. Some species of plants have finely 

 divided leaves when immersed in the water, but when stranded 

 by receding ponds, or when growing on muddy banks, they pro- 

 duce much less finely divided or even entire leaves (Fig. 204). 

 They may live either as hydrophytes or mesophytes. 



Harmful Characters. In contrast to useful characters there 

 are some structures whose development may reach the point of 

 inconvenience. In one extinct species, the Irish elk, the enormous 

 antlers were probably a handicap. The broad wings of many 

 butterflies make flight in a wind impossible, while others with 

 narrow wings and strong flight muscles are hindered but little. 



