V NATURAL SELECTION 119 



The Imjmicmce of Isolation. 



Isolation is no doubt an important aid to natural selection, 

 as shown by the fact that islands so often present a number 

 of peculiar species ; and the same thing is seen on the two 

 sides of a gi'eat mountain range or on opposite coasts of a 

 continent. The importance of isolation is twofold. In the 

 first place, it leads to a body of individuals of each species being- 

 limited in their range and thus subjected to uniform condi- 

 tions for long spaces of time. Both the direct action of the 

 cn-s-ironment and the natural selection of such varieties only 

 as are suited to the conditions, will, therefore, be able to 

 produce their full effect. In the second place, the process of 

 change will not be interfered with by intercrossing with other 

 individuals which are becoming adapted to somewhat different 

 conditions in an adjacent area. But this question of the 

 swamj^ing effects of intercrossing will be considered in another 

 chapter. 



Mr. Darwin was of opinion that, on the whole, the largeness 

 of the area occupied by a species was of more importance than 

 isolation, as a factor in the production of new species, and in 

 this I quite agree with him. It must, too, be remembered, 

 that isolation vn\\ often be produced in a continuous area 

 Avhenever a species becomes modified in accordance with varied 

 conditions or diverging habits. For example, a wide-ranging 

 species may in the northern and colder part of its area become 

 modified in one direction, and in the southern part in another 

 direction ; and though for a long time an intermediate form 

 may continue to exist in the intervening area, this Avill be 

 likely soon to die out, both because its numbers will be small, 

 and it will be more or less pressed upon in varying seasons by 

 the modified varieties, each better able to endure extremes of 

 climate. So, when one portion of a terrestrial species takes to 

 a more arboreal or to a more aquatic mode of life, the change 

 of habit itself leads to the isolation of each portion. Again, 

 as will be more fully explained in a future chapter, any 

 difference of haliits or of haunts usually leads to some modi- 

 fication of colour or marking, as a means of concealment from 

 enemies ; and there is reason to Ijelieve that this diff"erence will 

 be intensified by natural selection as a means of identification 



