Origin of Species 141 



who suggested that species became differentiated from 

 each other by the process to which he gave the name 

 of Natural Selection (87). An outline of the Dar- 

 winian theory may therefore introduce our discussion 

 as to the origin of species among insects. 



Natural Selection. — The first great fact on which 

 the theory rests is that variation on which we have 

 just dwelt ; the second is the " struggle for exist- 

 ence," which now requires a brief notice. In all 

 animals the number of young produced is greater, in 

 many far greater, than the number which survive. A 

 single female insect lays scores or hundreds of eggs, 

 but the number of individuals belonging to her species 

 does not increase twentyfold or a hundredfold ; it 

 remains approximately the same from year to year. 

 Some of the young hatched from these eggs fail to 

 find enough food, others may be drowned or may 

 fall victims to accidents, others may be killed and 

 devoured by creatures of prey. Of the many eggs 

 laid by a moth, for example, only a few give rise 

 to caterpillars which will pass safely through all 

 the changes and chances of their lives, so as to 

 become moths, capable, in their turn, of leaving off- 

 spring. 



What, then, decides the question which of these 

 young are to die and which to survive ? Those will 

 survive which have inherited or acquired the most 

 favourable variations, whether of structure, instinct, 

 or habit, while the less favourably endowed will be 

 exterminated. Thus the struggle for existence leads 

 to a "natural selection" for survival of those animals 

 most perfectly adapted to their surroundings and to 

 their manner of life. And it is believed that different 

 species have become slowly marked off from one 

 another, as, diverging more and more from a common 

 stock, each has attained more and more perfect corre- 



