A similar parallelism holds good with an allied yet very different class of facts.

It is an old and almost universal belief, founded on a considerable body of evidence, which I have elsewhere given, that slight changes in the conditions of life are beneficial to all living things.

We see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers, etc.

, from one soil or climate to another, and back again.

During the convalescence of animals, great benefit is derived from almost any change in their habits of life.