VI DIFFICULTIES AND OBJECTIONS 149 



soil, climate, and atmosphere widely difterent from those of 

 their native habitat. Thus, many alpine plants only found 

 near perpetual snow thrive well in our gardens at the level of 

 the sea ; as do the tritomas from the sultry plains of South 

 Africa, the yuccas from the arid hills of Texas and Mexico, arid 

 the fuchsias from the damp and dreary shores of the Straits of 

 Magellan. It has been well said that plants do not live 

 where they like, but Avhere they can ; and the same remark will 

 apply to the animal world. Horses and cattle run wild and 

 thrive both in North and South America ; rabbits, once con- 

 fined to the south of Europe, have established themselves in 

 our own country and in Australia ; while the domestic fowl, a 

 native of tropical India, thrives well in every part of the 

 temperate zone. 



If, then, we admit that when one portion of a species is 

 separated from the rest, there will necessarily be a slight 

 difference in the average characters of the two portions, it 

 does not follow that this difference has much if any effect 

 u])on the characteristics that are developed by a long period 

 of isolation,. In the first place, the difference itself will 

 necessarily be very slight unless there is an exceptional 

 amount of variability in the species ; and in the next place, 

 if the average characters of the species are the expression of 

 its exact adaptation to its Avhole environment, then, given 

 a precisely similar environment, and the isolated portion will 

 inevitably be brought back to the same average of characters. 

 But, as a matter of fact, it is impossible that the environment 

 of the isolated portion can be exactly like that of the bulk of 

 the species. It cannot be so physically, since no two separated 

 areas can be absolutely alike in climate and soil ; and even if 

 these are the same, the geographical features, size, contoui', and 

 relation to Avinds, seas, and rivers, Avould certainly differ. 

 Biologically, the differences are sure to be considerable. Thu 

 isolated portion of a species will almost always be in a much 

 smaller area than that occupied by the species as a whole, hence 

 it is at once in a different position as regards its own kind. 

 The proportions of all the other species of animals and plants 

 are also sure to difi'cr in the two areas, and some s})ecies will 

 almost always be absent in the smaller wliich are })resent in 

 the larger country. These differences will act and react on 



