Chap. V. LAWS OF VARIATION. 133 



into the zone of habitation of other species, often 

 acquiring in a very slight degree some of the characters 

 of such species, accords with our view that species of 

 all kinds are only well-marked and permanent varieties. 

 Thus the species of shells which are confined to tropical 

 and shallow seas are generally brighter-coloured than 

 those confined to cold and deeper seas. The birds 

 which are confined to continents are, according to Mr. 

 Gould, brighter-coloured than those of islands. The 

 insect-species confined to sea-coasts, as every collector 

 knows, are often brassy or lurid. Plants which live 

 exclusively on the sea-side are very apt to have fleshy 

 leaves. He who believes in the creation of each spe- 

 cies, will have to say that this shell, for instance, was 

 created with bright colours for a warm sea ; but that 

 this other shell became bright-coloured by variation 

 when it ranged into warmer or shallower waters. 



When a variation is of the slightest use to a being, 

 we cannot tell how much of it to attribute to the accu- 

 mulative action of natural selection, and how much to 

 the conditions of life. Thus, it is well known to fur- 

 riers that animals of the same species have thicker and 

 better fur the more severe the climate is under which 

 they have lived ; but who can tell how much of this 

 difference may be due to the warmest-clad individuals 

 having been favoured and preserved during many 

 generations, and how much to the direct action of 

 the severe climate? for it would appear that climate 

 has some direct action on the hair of our domestic 

 quadrupeds. 



Instances could be given of the same variety being 

 produced under conditions of life as different as can 

 well be conceived ; and, on the other hand, of different 

 varieties being produced from the same species under 

 the same conditions. Such facts show how indirectly 



