112 ZOOLOGICAL PHILOSOPHY 



which fill up the interval. Consequently there are also fine distinctions 

 between what we call species. 



It is obvious then that as regards the character and situation of the 

 substances which occupy the various parts of the earth's surface, 

 there exists a variety of environmental factors which induces a 

 corresponding variety in the shapes and structure of animals, inde- 

 pendent of that special variety which necessarily results from the 

 progress of the complexity of organisation in each animal. 



In every locality where animals can live, the conditions constituting 

 any one order of things remain the same for long periods : indeed they 

 alter so slowly that man cannot directly observe it. It is only by an 

 inspection of ancient monuments that he becomes convinced that in 

 each of these localities the order of things w^hich he now finds has 

 not always been existent ; he may thence infer that it will go on 

 changing. 



Races of animals living in any of these localities must then retain 

 their habits equally long : hence the apparent constancy of the races 

 that we call species, — a constancy which has raised in us the belief 

 that these races are as old as nature. 



But in the various habitable parts of the earth's surface, the character 

 and situation of places and climates constitute both for animals and 

 plants environmental influences of extreme variability. The animals 

 living in these various localities must therefore differ among themselves, 

 not only by reason of the state of complexity of organisation attained 

 in each race, but also by reason of the habits which each race is forced 

 to acquire ; thus when the observing naturalist travels over large 

 portions of the earth's surface and sees conspicuous changes occurring 

 in the environment, he invariably finds that the characters of species 

 undergo a corresponding change. 



Now the true principle to be noted in all this is as follows : 



1 . Every fairly considerable and permanent alteration in the environ- 

 ment of any race of animals works a real alteration in the needs of that 

 race. 



2. Every change in the needs of animals necessitates new activities 

 on their part for the satisfaction of those needs, and hence new habits. 



3. Every new need, necessitating new activities for its satisfaction, 

 requires the animal, either to make more frequent use of some of its 

 parts which it previously used less, and thus greatly to develop and 

 enlarge them ; or else to make use of entirely new parts, to which 

 the needs have imperceptibly given birth by efforts of its inner feeling ; 

 this I shall shortly prove by means of known facts. 



Thus to obtain a knowledge of the true causes of that great diversity 

 of shapes and habits found in the various known animals, we must 



