HISTORY OF ZOOLOGY. 55 



to satisfy itself upon the least favored of its compan- 

 ions. It is noticeable that in a severe epidemic cer- 

 tain men do not fall victims to the disease, because their 

 organization better withstands infection. Here applies 

 much better the term " survival of the fittest " which Spen- 

 cer has adopted in preference to the term "struggle for 

 existence." 



Instances of the Struggle for Existence. Although 

 the foregoing 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 (J\Ins deciimanus), which, at the beginning 

 of the last century, swarmed out from Asia, has since then 

 almost completely exterminated the house-rat (Mus rattus) 

 indigenous to Europe, and is pushing on to make exist- 

 ence impossible for it in other parts of the world. Sev- 

 eral European species of thistle have in the La Plata states 

 increased so enormously that they have in localities com- 

 pletely crowded out the native plants. Another European 

 plant (Hypochceris radicatd) has become in New Zealand a 

 weed overrunning everything. Certain races of men, like 

 the Dravidiau and Indian, die off to the same degree that 

 other races of men, like the Caucassian, Mongolian, and 

 Negro, spread. The more one attempts to seek explana- 

 tions of 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 learn to appreciate the method and results 

 of the Struggle for Existence. Then will he become con- 

 versant with many extremely interesting phenomena, which 

 immediately find an explanation through the doctrine of 

 the Struggle for Existence, which formerly were not in- 

 telligible. Islands lying in the midst of the ocean have 

 a disproportionately large number of species of wingless in- 

 sects, because the flying forms are easily carried out to sea. 

 For example, on the Kerguelen Islands, remarkably ex- 

 posed to storms, all the insects are wingless ; among them 



