314 GENERAL ZOOLOGY 



be supported in a given area. Darwin, however, used the term 

 struggle for existence in a larger sense to include such things as 

 the dependence of one organism on another and the success in 

 leaving offspring. In times of scarcity, two animals of the same 

 species may be truly said to struggle with each other to get food 

 and live. On the other hand, a plant on the edge of a desert 

 struggles for life against dryness. Mistletoe is parasitic on cer- 

 tain trees and therefore dependent upon them, but cannot be 

 said to struggle with the tree host, since too many parasites on 

 the same tree would kill it. But two seedling mistletoes on the 

 same branch may be said to struggle with each other. Since 

 mistletoe seeds are disseminated by birds, the mistletoe plant 

 is dependent on birds. The struggle for existence is a struggle 

 against various factors of environment, including other organisms. 



Survival of the Fittest. — Darwin believed that "variation is 

 generally related to the conditions of life to which each species 

 has been exposed during several successive generations," and 

 "that changed conditions act in two ways, directly on the whole 

 organization or on certain parts alone, and indirectly through 

 the reproductive system." Variations, favorable or otherwise, 

 are inherited. Darwin was uncertain as to what extent changed 

 conditions or food, climate, etc., acted in a definite manner to 

 produce variations, but he believed that "the effects have been 

 greater than can be proved by clear evidence." Success in the 

 struggle for existence over a period of time is heightened or 

 diminished according to whether favorable or unfavorable char- 

 acters are inherited. The fittest of each generation survive as a 

 result of the inheritance of favorable variations. 



Changes in environment alter the conditions determining 

 fitness. Thus the food supply of a large area, while averaging 

 constantly from year to year, may not be uniform throughout the 

 entire area, with the result that animals in the pursuit of food 

 migrate from famine zones to regions of plenty. Such immigra- 

 tion of a foreign species introduces a competitive factor for the 

 native species that in extreme cases might cause its extinction, 

 unless necessary readjustments in "fitness" were accomplished. 

 As Darwin notes, "when a variation is of the slightest use to 

 any being, we cannot tell how much to attribute to the accumula- 

 tive action of natural selection and how much to the definite 

 action of the conditions of life." 



