372 SICmFICANCE OF SEXUAL REPRODUCTION [V. 



immensely and have become almost a plague ; and the Eu- 

 ropean sparrow now thrives in Australia quite as well as with 

 us. But these instances do not prove that adaptation to ex- 

 ternal 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 surroundings. 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 ne- 

 cessity 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 difi'erent 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 locahties 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 sense of hearing or of smell, or were to suddenly acquire 

 a colour different from that which it now possesses, it would 

 become incapable of existence as a species, and would soon die 

 out. The same result would probably occur if any of its in- 

 ternal organs, such as the lungs or the liver, were suddenl}'^ 

 modified. Perhaps single individuals would still remain capable 

 of existence under these circumstances, but the whole species 

 would suffer a certain decline from the maximum development 

 of its powers of resistance, and would thus become extinct. 

 The sudden transformation of a species appears to me to be 

 inconceivable from a physiological point of view, at any rate in 

 animals. 



Hence the transformation of a species can only take place by 

 the smallest steps, and must depend upon the accumulation of 

 those differences which characterise individuals, or, as we call 

 them, 'individual differences.' There is no doubt that these 

 diff'crences are always present, and thus, at first sight, it appears 

 to be simply a matter of course that they will afford the material 

 by means of which natural selection produces new forms of life. 

 But the case is not so simple as it appeared to be until recently ; 



