ARTIFICIAL AND NATURAL SELECTION 41 



introduced into Victoria in i860, became so prolific 

 as to overrun the greater portion of the colony, and 

 great sums of money were expended in endeavouring 

 to exterminate it.. 



B. The Numbers are Stationary as a Whole. — 

 Some forms increase while others diminish in num- 

 bers, and hence there is not actually this rapid 

 increase of adult forms. Enormous numbers are 

 devoured as food in their early stages, the seeds of 

 plants being eaten by birds, and the young of 

 various animals by other animals. Many animals 

 again have their numbers kept down by parasites ; 

 for instance, the caterpillar of the large garden 

 white butterfly is peculiarly liable to attacks from 

 the ichneumon fly, which lays its eggs in the body 

 of the caterpillar ; and out of 533 larvae collected 

 by Mr. Poulton in 1888, 422 full-fed caterpillars 

 died from the presence of ichneumon grubs — i.e., four 

 out of five perished from this cause alone. 



C. The Struggle for Existence. — Every single 

 organic being may be said to be striving to the 

 utmost of its power to increase its numbers, while 

 the vast majority of animals and plants that come 

 into the world are doomed to die early. For instance, 

 in the case of an annual plant producing 1000 seeds 

 — which is no very large estimate — if the numbers 

 remain stationary, only one of these 1000 can on 

 the average come to maturity ; and it may be said 

 to struggle with plants of the same or other kinds 

 which already clothe the ground. In fact, " all the 

 plants of a country are at war with each other." 



Again, the introduction of goats into St. Helena led 



