V . NATURAL SELECTION 119 



The Importance of Isolation. 



Isolation is no doubt an important aid to natm^al 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 gTcat 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 

 enWronment and the natural selection of such varieties only 

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

 produce their fidl effect. In the second place, the jDrocess of 

 change will not be interfered with by intercrossing with other 

 individuals ^vhich are becoming adapted to somewhat different 

 conditions in an adjacent area. But this question of the 

 swamping eflects of intercrossing ^nll be considered in another 

 chapter. 



Mr. Dar^vin 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 mth him. It must, too, be remembered, 

 that isolation will often be produced in a continuous area 

 whenever 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 ^vill be 

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

 and it ^^^ill 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 ai'boreal or to a more aquatic mode of life, the change 

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

 as \^411 be more fully exj^lained in a future chapter, any 

 difference of habits 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 believe that this difference will 

 be intensified by natural selection as a means of identification 



