v.] IN THE THEORY OF NATURAL SELECTION. 27 1 



up which ensures that only those out of the existing variations 

 are preserved, which correspond in the highest degree to the 

 changed conditions of Hfe. By continued selection in the same 

 direction the deviations from the type, although at first very 

 insignificant, are accumulated and increased until they become 

 specific differences. 



I should wish to assert more definitely than Darwin has done, 

 that alterations in the conditions of life, together with changes 

 in the organism itself, must have advanced very gradually and 

 by the smallest steps, in such a way that, at each period in 

 the whole process of transformation, the species has remained 

 sufficiently adapted to the surrounding conditions. An abrupt 

 transformation of a species is inconceivable, because it would 

 render the species incapable of existence. If the whole or- 

 ganization of an animal depends upon adaptation, if the animal 

 body is, as it were, an extremely complex combination of new 

 and old adaptations, it would be a highly remarkable coincidence 

 if, after any sudden alteration occurring simultaneously in many 

 parts of the body, all these parts were changed in such a manner 

 that they again formed a whole which exactly corresponded to 

 the altered external conditions. Those who assume the ex- 

 istence 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 incapable 

 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 gfecial epoch, and which 

 at its close migrated to the Alpine mountains and to the far 

 north, and which have remained unaltered under the ap- 

 parently diverse conditions 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 abun- 

 dantly and remains unchanged in this locality ; the European 

 frogs, which were introduced into Madeira, have increased 



