E. Forbes speaks confidently that shells at their southern limit, and when living in shallow water, are more brightly coloured than those of the same species further north or from greater depths.  Gould believes that birds of the same species are more brightly coloured under a clear atmosphere, than when living on islands or near the coast.  So with insects, Wollaston is convinced that residence near the sea affects their colours.  Moquin-Tandon gives a list of plants which when growing near the sea-shore have their leaves in some degree fleshy, though not elsewhere fleshy.  Several other such cases could be given.

The fact of varieties of one species, when they range 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 species, 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 accumulative action of natural selection, and how much to the conditions of life.  Thus, it is well known to furriers 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 the conditions of life must act.  Again, innumerable instances are known to every naturalist of species keeping true, or not varying at all, although living under the most opposite climates.  Such considerations as these incline me to lay very little weight on the direct action of the conditions of life.  Indirectly, as already remarked, they seem to play an important part in affecting the reproductive system, and in thus inducing variability; and natural selection will then accumulate all profitable variations, however slight, until they become plainly developed and appreciable by us.