THE EVOLUTION OF ANIMAL LIFE 



of the environment or of the organism but appear spontaneously and in all 

 directions. By chance, some of the variations will be advantageous in the 

 struggle for existence, and others will be unrelated to survival or will be 

 detrimental. Natural selection involves the action of the environment in 

 selecting for survival those forms which, by chance, are best adapted to 

 environmental conditions, and in eliminating those less well adapted. This 

 results in the survival of the fittest, in terms of any specific environment. 

 If the characteristics of the environment change, a new process of selection 

 begins and a new group of organisms is selected, with modifications in adapta- 

 tion to the changed environment. 



In The Origin of Species, Darwin cites example after example of observations 

 supporting these principles. In his accounts of the environmental relation- 

 ships of organisms, Darwin anticipates many of the important generalizations 

 that have arisen from modern ecological studies. Much of our discussion in 

 Chapter 19 states in specialized terminology the principles and conclusions 

 that Darwin adduced in support of his theorv of evolution. Consider, for ex- 

 ample, Darwin's concept of the "struggle for existence." Since the capacity 

 for reproduction is restricted bv checks upon increase, relatively few of the 

 individuals that begin life in any generation will reach maturity. Each in- 

 dividual, therefore, must engage in a fight for survival. As Darwin conceived 

 it, this struggle for existence is seldom an actual conflict, although this may 

 be involved when animals fight with one another for mates or for food. He 

 thought that the struggle would be most acute between individuals of the 

 same species, since these compete for the same conditions of life; or between 

 different species using the same food, as when insects devour the food of graz- 

 ing mammals. It is important to bear in mind that Darwin used the term 

 struggle in a metaphorical sense. In the vast majority of cases there is nothing 

 that can be called a struggle in the sense of actual conflict. Metaphorically, 

 however, it can be said that the trees of a forest, competing for soil nutrients 

 and for light, ''struggle" to exist or "fight" for life. Darwin concluded that 

 such a struggle, in one or more of its aspects, is ever-recurring for all 

 organisms, although it is intermittent and may not act for considerable 

 periods in the life of any individual. The elements of Darwin's struggle for 

 existence are implicit in the broad modern concept of competition (pp. 609- 

 614). 



The modern views of variation and heredity have been presented in 

 Chapter 6. Heredity has been defined as the tendency of individuals to 

 resemble their ancestors and relatives, and variation as the tendency of 

 individuals related by descent to differ in various ways. The two are 

 intimatelv connected as different expressions of the reproductive and develop- 

 mental processes. Darwin observed that the members of species varied, and 

 he believed that many of these variations, small though they might be in 

 many instances, were inherited. He was interested in heredity and variation 

 as such and studied them intensively; but so far as they concerned natural 

 selection, it was not necessary to explain them. His argument was: given 



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