420 THE METHOD OF ORGANIC EVOLUTION. 



wlio luive given special attention to the relations of living organisms to 

 each other and to their inorganic environment will be that of an aca- 

 demic discussion, dealing to a large extent with Avords rather than with 

 the actual facts of nature. The author's main point that si)eeies form 

 a discontinuous series, and that specific differences can not, therefore, 

 have been produced by any action of the environment, because that 

 environment is continuous — an argument which, as we have seen, he 

 dwells ui)on and reiterates with emphasis and i)ersistency — rests wholly 

 upon the obvious IVillacies that in each single locality the environment 

 of every species found there is the same, aud that all change of envi- 

 ronment, whether in space or time, is continuous. To take this latter 

 })oiiit first, nothing can be more abru])t than the change often due to 

 diversity of soil, a sharp line dividing a pine or heather clad moor from 

 calcareous hills; or to differences of level, as from a marshy plain to dry 

 u[)lands; or, for aquatic animals, fnmi the open sea to an estuary, or 

 from a uontidal stream to an isolated pond. And when, in the course 

 of geological time, an island is separated from a continent, or volcanic 

 outbursts build up oceanic islands, the immigrants whicli n^ach such 

 islands undergo a. change of environment which is in a higli degree 

 discontinuous. 



pjven more important, perhaps, is the fact that everywhere the 

 environment as a whole is made up of an unlimited number of sub- 

 environments, each of which alone, or nearly alone, affects a single 

 si)ecies, as familiarly included in the term "their conditions of exist- 

 ence." The mole and the hedgehog may live together in the saujc 

 general environment, yet their actual environments are very dilferent, 

 owing to their different kinds of ibod, habits, and enemies. The same 

 thing api)lies to the rabbit and the hare, the rook and the crow, the 

 ring snake and the viper; aud 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 lias its own "environment," to which it must be adapted in order 

 to maintain its existence. Many species, liowever, live, as it were, on 

 the borders of two distinct environments, as when they obtain differ- 

 ent kinds of food at different i>eriods, being then exposed to different 

 enemies and varied (dimatic effects. In such cases, it is easy to see 

 that a small modification of structure might enable them advanta- 

 geously to cliange their habits, and thus obtain what would be practi- 

 cally a different environment. This is well seen in those closely allied 

 species whicli have somewhat different modes of life, as the meadow 

 pipit (A«///»-s' j>>7//r».s'/.s-) aud the tree i[)i\)it {An fliK.s <(rborei(s), the former 

 having a long, nearly straight claw^ to the hind toe, a more slender bill, 

 and a rather greener tinge of coloring, a llmodifications suited to its 

 different habits and distinct physical surroundings. Here we have an 

 example in nature of how^ environments, even when continuous as a 



