IN THE THEOEY OF NATURAL SELECTION. 265 



changed in such a manner that they again formed a whole which 

 exactly corresponded to the altered external conditions. Those who 

 assume the existence of such a sudden transformation overlook the 

 fact that everything in the animal body is exactly calculated to 

 maintain the existence of the species, and that it is just sufficient 

 for this purpose ; and they forget that the minutest change in the 

 least important organ may be enough to render the species in- 

 capable of existence. 



It may perhaps be objected that the case is different in plants, as 

 is proved by the American weeds which have spread all over 

 Europe, or the European plants which have become naturalized in 

 Australia. Reference might also be made to the plants which 

 inhabited the plains during the glacial epoch, and which at its 

 close migrated to the Alpine mountains and to the far north, and 

 which have remained unaltered under the apparently diverse con- 

 ditions of life to which they have been subjected for so long a 

 time. Similar instances may also be found among animals. The 

 rabbit, which was brought by sailors to the Atlantic island of Porto 

 Santo, has bred abundantly and remains unchanged in this locality ; 

 the European frogs, which were introduced into Madeira, have in- 

 creased immensely and have become almost a plague ; and the 

 European sparrow now thrives in Australia quite as well as with us. 

 But these instances do not prove that adaptation to external 

 conditions of life is not of primary importance ; they do not prove 

 that an organism which is adapted to a certain environment will, 

 when unmodified, remain capable of existence amid new surround- 

 ings. They only prove that the above-mentioned species found 

 in those countries the same conditions of life as at home, or at 

 least that they met with conditions to which their organization 

 could be subjected without the necessity for modification. Not 

 every new environment includes such changed conditions as will be 

 effective in modifying every species of plant or animal. The rabbit 

 of Porto Santo certainly feeds on herbs different from those which 

 form the food of its relations in Europe, but such a change does 

 not mean an effective alteration in the conditions under which this 

 species lives, for the herbs in both localities are equally well suited 

 to the needs of the animal. 



But if we suppose that the wild rabbit, occurring in Europe, were 

 to suddenly lose but a trifle of its wariness, its keen sight, its fine 



