Variation under Domestication

Our varieties of older domesticated plants and animals generally differ much more from each other than do the undomesticated ones.

Our domesticated plants and animals have a vast diversity from all the ages under the most different climates and treatment.

This is caused by our domestic productions creating non-uniform conditions in life compared to the wild ones.

Andrew Knight thinks that this variability is partly connected with the excess of food.

Organic beings must be exposed for several generations to new conditions to cause any appreciable amount of variation.

When the organisation has begun to vary, it continues to vary for many generations.

Variations do not stop for cultivated organisms.

• Our oldest cultivated plants, such as wheat, still often yield new varieties.
• Our oldest domesticated animals still rapidly improve or change.

At what period of life does the variability happen?

Geoffroy St. Hilaire’s experiments show that unnatural treatment of the embryo causes monstrosities.

I strongly believe that the most frequent cause of variability is the male and female reproductive elements being affected prior to the act of conception.

The chief reason is the effect of domestication on the reproductive functions.

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

It is easy to:

• tame an animal
• get it to breed freely under confinement

Some animals will not breed in confinement. This is generally due to vitiated instincts.

Many cultivated plants are very vigourous yet never seed!

In a few cases, very trifling changes will cause them to seed, such as adding a little more water at some period of growth.

The laws which determine the reproduction of confined animals is so singular.

Carnivorous animals, even from the tropics, breed in the UK freely under confinement, except for the plantigrades or the bear family.

Whereas, carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs.

Many exotic plants have pollen utterly worthless similar to sterile hybrids.

• On one hand, weak and sickly domesticated animals and plants can breed freely under confinement.
• On the other hand, healthy undomesticated individuals are unable to breed when domesticated due to unperceived causes

When this system works under confinement, it:

• does not do so regularly
• does not produce offspring as perfectly like their parents

Sterility is the bane of horticulture.

But we owe variability to the same cause which produces sterility.

Variability is the source of all the choicest productions of the garden.

Some organisms will breed most freely under the most unnatural conditions (e.g. a rabbit and ferret kept in hutches).

• This shows that their reproductive system has not been affected.

Likewise, some animals and plants withstand domestication or cultivation.

• They vary very slightly, hardly more than in a wild state.

‘Sporting plants’ are a single bud or offset which suddenly assumes a new and different character from the rest of the plant.

• Such buds can be propagated by grafting, &c., and sometimes by seed.

These ‘sports’ are extremely rare under nature, but common under cultivation.

Here, the treatment of the parent has affected a bud or offset, and not the ovules or pollen.

But most physiologists think that there is no essential difference between a bud and an ovule in their earliest stages of formation.

‘Sports’ support my view that variability is due to the ovules or pollen, or to both.

• These are affected by the treatment of the parent prior to the act of conception.

These cases show that variation is not necessarily connected with the act of generation.

The following sometimes differ considerably from each other:

• seedlings from the same fruit
• the young of the same litter

This is even though both the young and the parents, as Muller has remarked, have been exposed to exactly the same conditions of life.

This shows how unimportant the direct effects of the conditions of life are compared with:

• the laws of reproduction
• the laws of growth
• the laws of inheritance

Had the action of the conditions been direct, if any of the young had varied, all would probably have varied in the same way.

It is most difficult to judge how much, in the case of any variation, we should attribute to the direct action of heat, moisture, light, food, &c.

With animals, such agencies have produced very little direct effect, though apparently more in the case of plants.

This is why Mr. Buckman’s recent experiments on plants are extremely valuable.

When all or nearly all the individuals exposed to certain conditions are affected in the same way, the change at first appears to be directly due to such conditions.

But in some cases, quite opposite conditions produce similar changes of structure.

Nevertheless, I think that some slight amount of change may be attributed to the direct action of the conditions of life, such as:

• increased size from amount of food
• colour from particular kinds of food and from light
• the thickness of fur from climate