54 GENERAL PRINCIPLES OF ZOOLOGY. 



velopment of this animal throng there is no room on the 

 earth ; for even if we compute upon the basis of a slowly- 

 multiplying animal, as, for example, the elephant, and 

 assume that all the progeny live and reproduce normally, it 

 would only be a few centuries before the entire earth would 

 be occupied by herds of elephants. In order to preserve 

 the equilibrium in the household of nature there must per- 

 ish great numbers of unfertilized and fertilized eggs, as 

 well as young animals and many that are mature but have 

 not yet attained their physiological destiny. Many exist- 

 ences will undoubtedly be blotted out by purely accidental 

 causes ; yet on the whole those individuals which are best 

 protected will best withstand pressing danger. Slight su- 

 periorities in structure will be of importance in this strug- 

 gle for existence, and the possessors of these will gain an 

 advantage over their companions of the same species, just 

 as in domestication each character which happened to be 

 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 increase by summation, while 

 destruction overtakes the unsuitable varieties. 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 existence" is a figurative one, for only in 

 rare cases does an active conscious struggle take place in 

 regard to the prospects of an animal's existence ; as 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 competitors for his booty is best pro- 

 vided in times of limited food-supply. Much more com- 

 mon 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 causes the voracity of the enemy 



