462 AN INTRODUCTION TO ZOOLOGY 



fight so well, see so well, are not so cunning, etc., will be exterminated 

 ruthlessly. The more animals produced the more fierce this 

 " struggle for existence " and the larger the number of individuals 

 blotted out, and we can see that this all becomes intensified if, 

 instead of the simple case imagined, we have, as we have in fact, 

 large numbers of different animals on the same area. 



The whole of the external surroundings of an individual, the 

 earth or water, the other animals, the plants, the climate, the 

 weather and all the many factors of the external world that affect 

 an animal even in the remotest way, we include in the one term 

 " environment. ' ' Thus the environment of an animal is every external 

 influence that plays upon it from the moment it starts life as a 

 fertilised egg until its death. In order to survive in the struggle for 

 existence the animal must be suited to its surroundings in its struc- 

 ture and habits. Thus it would be useless for a cat to develop a fin 

 unless it at the same tune altered its mode of life, and so on. We 

 put this in another way by saying that an animal must be " adapted 

 to its environment." A character or structure is called adaptive 

 when it is of obvious use to the possessor. An animal that is well 

 adapted to a certain environment is said to be fitted to it. Note 

 the scientific use of the word " fit," so often misunderstood or 

 misused in popular writings. When we say an animal is "fit " we 

 imply no physical, mental or moral superiority whatever ; we simply 

 mean that it is adapted to its environment. Should the environ- 

 ment change, the animal that was fit probably becomes unfit. We 

 have realised that in the struggle for existence it is the unfit that 

 are eliminated, and so indirectly the fittest are selected. Hence 

 the philosopher Herbert Spencer, who arrived at much the same 

 conclusions as Darwin from more theoretical reasoning, termed 

 natural selection " the survival of the fittest." 



Thus we have glanced quite briefly at the three main factors 

 that Darwin recognised, Variation, Heredity and Natural Selection. 



The struggle for existence is not so simple a process as we 

 assumed in the imaginary example of the animals on the island, and 

 various entirely different forms have an enormous indirect influence 

 and dependence upon one another. To illustrate this we may take 

 two well-known examples. It is not at first sight obvious why the 

 crop of red clover (Trifolium pratense) should be in any way de- 

 pendent upon cats, yet it is to a certain extent. Red clover is 

 fertilised almost entirely by bees ; the number of bees in a neighbour- 

 hood (excluding carefully guarded hives) depends on the mice, 

 which destroy the bees' nests, eggs and young ; the number of mice 

 is largely kept down by cats (owls also play a part). 



Many years ago goats were introduced into St. Helena, which 



