286 MORPHOLOGY 



the doctrine of evolution. Whether the theory stands or falls as an 

 explanation of the origin of species, its supreme importance in the 

 history of biology demands that it be understood by all students of 

 plants and animals. The bare outline of the theory is as follows: 



The theoretical " ratio of increase " of plants and animals is far be- 

 yond their actual increase. If an annual plant should produce two seeds, 

 and each seed should " fulfill its mission," there would be two plants 

 in the second season, four in the third, eight in the fourth, and so on 

 in geometrical ratio, until in comparatively few years there would be 

 many millions of descendants from a single individual, enough to popu- 

 late the whole earth. If this ratio of increase be applied to the myriads 

 of plants and animals of many kinds, the result would be a tremendous 

 competition for space and food, a competition which has been called 

 " the struggle for existence." Since in general the number of adult 

 plants and animals is no greater in one season than in the preceding, it 

 is evident that the " struggle " results in a tremendous destruction of 

 individuals. This leads to the striking conclusion that " death is the 

 rule, and life the exception." 



In considering this enormous waste of living forms, Darwin concluded 

 that the survivors of the " struggle " must be better situated or equipped 

 than their less fortunate fellows, and that the competition resulted in 

 what Spencer afterwards called " the survival of the fittest," which is 

 another way of saying " the destruction of the unfit." 



The idea that two plants from the same parent might be differently 

 equipped, led to the observation of the facts of variation. No two 

 individuals of the same species, even from the same parent, are alike 

 in every detail; and the variations range from very minute ones to very 

 large and striking ones. It was concluded that there must be a selection 

 from among these variants of those best suited to the conditions of living. 

 There was no attempt at this time to search for the cause of variation; 

 it was simply accepted as a fact which makes evolution possible. 



The actual demonstration of the use that can be made of variations 

 was obtained by Darwin from the operations of plant and animal breed- 

 ers, who had long changed plants and animals under domestication. 

 Some of these changes had been so extensive that it was difficult to be- 

 lieve that the wild form and the highly cultivated form were the same 

 species. In fact, had they both been found growing wild they would 

 probably have been described as two species. The process of the 

 breeders was to select from the variants those which best suited their 



