356 STUDIES, SCIENTIFIC AND SOCIAL chap. 



unlimited number of sub-environments, each of which 

 alone, or nearly alone, affects a single species, as familiarly 

 included in the term, " their conditions of existence." The 

 mole and the hedgehog may live together in the same 

 general environment, yet their actual environments are 

 very different owing to their different kinds of food, 

 habits, and enemies. The same thing applies to the 

 rabbit and the hare, the rook and the crow, the ring- 

 snake and the viper; and still more when we look at 

 animals of greater diversity, as the otter and the badger, 

 the dung-beetle and the cockchafer, and a hundred others 

 that might be quoted. Now, though all these creatures 

 may be found together in the same area, each of them has 

 its own " environment," to w^hich it must be adapted in 

 order to maintain its existence. Many species, however, 

 live, as it were, on the borders of two distinct environ- 

 ments, as when they obtain different kinds of food at 

 different periods, being then exposed to different enemies 

 and varied climatic effects. In such cases, it is easy to 

 see that a small modification of structure might enable 

 them advantageously to change their habits, and thus 

 obtain what would be practically a different environment. 

 This is well seen in those closely allied species which have 

 somewhat different modes of life — as the meadow pipit 

 {Anthus pratensis) and the tree pipit (Anthus arhoreus) — 

 the former having a long, nearly straight claw to the hind 

 toe, a more slender bill, and a rather greener tinge of 

 colouring, all modifications suited to its different habits 

 and distinct physical surroundings. Here we have an 

 example in nature of how environments, even when 

 continuous as a whole, may become quite discontinuous in 

 relation to two species differing in very slight characters. 

 Darwin dwelt much upon this phenomenon, of new species 

 being formed when any body of individuals seized upon 

 vacant places in the economy of nature, and by means of 

 comparatively slight variations became adapted to it. It 

 is what we see everywhere in the world around us. 



