EISTORT OF ZOOLOGY. 45 



tlio entire earth would bo occupied by herds of elephants. In 

 order to preserve the equilibrium in nature great numljers of 

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

 that 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 bo 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 nunierous 

 varieties that appear the fittest will survive, and hi the course of 

 many generations the fortunate variations will increase by sum- 

 mation, while destruction overtakes tho 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 '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 wliich by 

 means of his bodily strength is best able to struggle with his com- 

 jietitors for his prey is best 2'>rovided in times of limited food- 

 supjoly. Much more common is the unconscious struggle: each 

 man who attains a more favorable jjosition 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, tho 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 tlie Sfrvggle for Existence — ^ Although tho fore- 

 ffoing general considerations suffice to show that the struggle for 

 existence plays a very jn'ominent role in the organic world, yet on 

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

 a few concrete examples. The migratory rat {Mtis decumanus), 

 which swarmed out from Asia at the beginning of the eighteenth 

 century, has since then almost completely exterminated tho house- 

 rat {Mus rattus) in Eurojie, and has made existence imiDossible for 



