Chap. V. 
LAWS OF VAKIATION. 
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 slighest 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 
