CHAP. VII.] THEORY OF NATURAL SELECTION. 167 



best be given by an illustration. In every meadow in England in 

 which trees grow, we see the lower branches trimmed or planed 

 to an exact level by the browsing of the horses or cattle ; and what 

 advantage would it be, for instance, to sheep, if kept there, to 

 acquire slightly longer necks 1 In every district some one kind of 

 animal will almost certainly be able to browse higher than the 

 others ; and it is almost equally certain that this one kind alone 

 could have its neck elongated for this purpose, through natural 

 selection and the effects of increased use. In S. Africa the com- 

 petition for browsing on the higher branches of the acacias and 

 other trees must be between giraffe and giraffe, and not with the 

 other ungulate animals. 



Why, in other quarters of the world, various animals belonging 

 to this same order have not acquired either an elongated neck or 

 a proboscis, cannot be distinctly answered ; but it is as unreason- 

 able to expect a distinct answer to such a question, as why some 

 event in the history of mankind did not occur in one country, 

 whilst it did in another. We are ignorant with respect to the 

 conditions which determine the numbers and range of each 

 species ; and we cannot even conjecture what changes of structure 

 would be favourable to its increase in some new country. We 

 can, however, see in a general manner that various causes might 

 have interfered with the development of a long neck or proboscis. 

 To reach the foliage of a considerable height (without climbing, 

 for which hoofed animals are singularly ill-constructed) implies 

 greatly increased bulk of body ; and we know that some areas 

 support singularly few large quadrupeds, for instance S. America, 

 though it is so luxuriant ; whilst S. Africa abounds with them to 

 an unparalleled degree. Why this should be so, we do not know ; 

 nor why the later tertiary periods should have been much more 

 favourable for their existence than the present time. Whatever 

 the causes may have been, we can see that certain districts and 

 times would have been much more favourable than others for the 

 development of so large a quadruped as the giraffe. 



In order that an animal should acquire some structure specially 

 and largely developed, it is almost indispensable that several other 

 parts should be modified and co-adapted. Although every part of 

 the body varies slightly, it does not follow that the necessary parts 

 should always vary in the right direction and to the right degree. 

 With the different species of our domesticated animals we know 

 that the parts vary in a different manner and degree ; and that 

 some species are much more variable than others. Even if the 

 fitting variations did arise, it does not follow that natural selection 

 would be able to act on them, and produce a structure which 

 apparently would be beneficial to the species. For instance, if the 

 number of individual? existing in a country is determined chiefly 



