Tones up

Usage examples of conditions.

When we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, I think we are driven to conclude that this greater variability is simply due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent-species have been exposed under nature.

If it could be shown that our domestic varieties manifested a strong tendency to reversion,--that is, to lose their acquired characters, whilst kept under unchanged conditions, and whilst kept in a considerable body, so that free intercrossing might check, by blending together, any slight deviations of structure, in such case, I grant that we could deduce nothing from domestic varieties in regard to species.

In the case of the misseltoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of the plant itself.

Naturalists continually refer to external conditions, such as climate, food, &c.

It seems pretty clear that organic beings must be exposed during several generations to the new conditions of life to cause any appreciable amount of variation.

I may add, that as some organisms will breed most freely under the most unnatural conditions (for instance, the rabbit and ferret kept in hutches), showing that their reproductive system has not been thus affected.

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.

Nevertheless some slight amount of change may, I think, be attributed to the direct action of the conditions of life--as, in some cases, increased size from amount of food, colour from particular kinds of food and from light, and perhaps the thickness of fur from climate.

I may add, that when under nature the conditions of life do change, variations and reversions of character probably do occur.

Some little effect may, perhaps, be attributed to the direct action of the external conditions of life, and some little to habit.

The keeping of a large number of individuals of a species in any country requires that the species should be placed under favourable conditions of life, so as to breed freely in that country.

I believe that the conditions of life, from their action on the reproductive system, are so far of the highest importance as causing variability.

Something may be attributed to the direct action of the conditions of life.

Some authors use the term 'variation' in a technical sense, as implying a modification directly due to the physical conditions of life.

These facts seem to be very perplexing, for they seem to show that this kind of variability is independent of the conditions of life.