Laws of Variation

The variations are:

• so common and multiform in organic domesticed beings
• less common in undomesticated ones

These did not happen all by chance.

Some authors believe variation to be the function of the reproductive system to produce individual differences as to make the child like its parents.

I think that the deviations of structure are due to the nature of the conditions of life to which the parents and ancestors have been exposed to. This is because of:

• the much greater variability
• the greater frequency of monstrosities under domestication than under nature

The reproductive system is susceptible to changes in the conditions of life.

The varying or plastic condition of the offspring is due to this system being functionally disturbed in the parents.

The male and female sexual elements are affected before that union takes place which is to form a new being.

In the case of ‘sporting’ plants, the bud in its earliest condition does not differ from an ovule.

• The bud is alone affected.

But we do not know why some parts vary in the offspring when the reproductive system is disturbed.*

Nevertheless, we can here and there dimly catch a faint ray of light, and we may feel sure that there must be some cause for each deviation of structure, however slight.

It is extremely hard to show how differences of climate, food, etc. produce differences on any being.

I think that the effect is:

• extremely small in the case of animals
• larger in plants

Such differences cannot have produced the different and complex co-adaptations of structure between one organic being and another throughout nature.

Some little influence may be attributed to climate, food, &c.

This is why 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.

A variety of a species that enters and lives in the zone of habitation of another species often acquires in a very slight degree some of the characters of such species.

This is consistent with my view that all species are only well-marked and permanent varieties.

Thus:

• the species of shells 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 are often brassy or lurid
• plants which live exclusively on the sea-side often have fleshy leaves.

When a variation is of the slightest use to a being, we cannot tell how much of it is from:

• the accumulative action of natural selection
• the conditions of life.

Thus, animals of the same species have thicker and better fur the more severe their climate is.

But how can we 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?

Climate has some direct action on the hair.

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.

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.