BISTORT OF ZOOLOGY. 45 



the entire earth would be occupied by herds of elephants. In 

 order to preserve the equilibrium in nature great numbers of 

 unfertilized and fertilized eggs, as well as young animals and many 

 thafc are mature but have not yet attained their physiological 

 destiny, must perish. Many individuals will undoubtedly be 

 blotted out by purely 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 or is fancied to be useful to 

 man insures 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 sum- 

 mation, while destruction overtakes the unsuitable varieties. Thus 

 will arise new forms, which owe their existence to ' natural selec- 

 tion in the struggle for existence/ 



The < Struggle for Existence.' The expression i struggle for 

 existence ' is figurative, for only in rare cases does an active con- 

 scious 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 his com- 

 petitors 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 com- 

 panions. 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 exist- 

 ence/ is better. 



Instances of the Struggle for Existence. Although the fore- 

 going general considerations suffice to show that the struggle for 

 existence plays a very prominent role in the organic world, yet on 

 account of the importance of this feature it will be illustrated by 

 a few concrete examples. The migratory rat (Mus decumanus), 

 which swarmed out from Asia at the beginning of the eighteenth 

 century, has since then almost completely exterminated the house- 

 rat ( Mus rattus) in Europe, and has made existence impossible for 



