36 GENERAL PRINCIPLES OF ZOOLOGY 



be blotted out by accidental causes; yet on the whole those individuals 

 which are best protected will best withstand adverse conditions. Slight 

 superiority in structure will be of importance in this struggle for existence, 

 and the possessors of this will gain an advantage over their companions 

 of the same species, just as in domestication each character which is 

 useful to man is of advantage to the possessor. Among the numerous 

 varieties that appear the fittest will survive, and in the course of many 

 generations the fortunate variations will increase by summation, while 

 destruction overtakes the unfit. Thus will arise new forms, which owe 

 their existence to 'natural selection in the struggle for existence.' 



The 'Struggle for Existence.' The expression 'struggle for exis- 

 tence' is figurative, for only rarely does a conscious struggle decide the 

 question of an animal's existence; for example, in the case of the beasts 

 of prey, that one which by means of his bodily strength is best able to 

 struggle with Ids competitors for his prey is best provided in times of 

 limited food-supply. Much more common is the unconscious struggle: 

 each man who attains a more favorable position by special intelligence 

 and energy, limits to an equal degree the conditions of life for many of his 

 fellow men, however much he may interest himself in humanity. The 

 prey, which by special craft' or swiftness escapes the pursuer, turns the 

 enemy upon the less favored of its companions. It is noticeable that in 

 severe epidemics certain men do not fall victims to the disease, because 

 their organization better withstands infection. Here the term 'survival 

 of the fittest,' which Spencer has adopted in preference to 'struggle for 

 existence,' is better. 



Instances of the Struggle for Existence. Although the foregoing 



suffices to show that the struggle for existence plays a very prominent 



role, yet on account of the importance of this feature it will be illustrated 



by a few concrete examples. The brown rat (Mus decumanus) , which 



swarmed out from Asia at the beginning of the eighteenth century, has 



almost completely exterminated the black or house-rat (Mus rattus) in 



Europe, and has made existence impossible for it in other parts of the 



world. Several European species of thistle have increased so enormously 



in the La Plata states that they have in places completely crowded out the 



native plants. Another European plant (Hypochoeris radicata) has 



become a weed, overrunning everything in New Zealand. Certain races 



of men, like the Dravidian and Indian, die off to the same degree that 



other races of men, like the Caucasian, Mongolian, and Negro, spread. 



The more one attempts to explain that endlessly complicated web of the 



relations of animals to one another, the relations of animals to plants and 



to climatic conditions, as Darwin has done, so much the more does he 



