The character of the soil has a marked influence in inducing somatogenetic variations. In France an experimenter collected seed from the wild radish and sowed one lot in heavy soil in the country, while another lot was sown by him in the dry, light soil near the Museum of Natural History in Paris. The radish “roots” grown in these two places presented marked differences in color and form. Those grown in Paris were either of a rose or white color and elongated; while those from the country were violet, dark-brown or nearly black in color, and more rounded than the former.
In the summer of 1847 Professor Buckman gathered seed from wild parsnips, and sowed them in the spring of 1848 under changed conditions of life. Most of the plants grown from these seeds were like the wild parsnips, but some of them developed the light-green color and hairless, smooth appearance characteristic of the cultivated plant. The roots also were found to be more fleshy than those of the wild variety.
Peas and squashes, when grown in different soils, often show remarkable variations.
There is one species of shrimp that inhabits brackish water, and another that lives in water which is much more salt. These crustaceans differ from one another in the character of the spines they bear and in the form of the tail-lobes. They have been regarded as distinct species, and yet either of them can be transformed into the other in the course of a few generations, by gradually altering the saline conditions of the water.
For a long while the siredon and amblystoma were regarded as being distinct genera of amphibians. Siredon was looked upon as a permanent gill-breather, while amblystoma passes through a metamorphosis and becomes a permanent lung-breather. It is now known that the former can change into the latter. If there is plenty of water the siredon remains indefinitely a gill-breather and reproduces freely; but when the water dries up it changes into the lung-breathing amblystoma. These two cases illustrate very well the power of environment to modify the development of organic forms.
As to “use” and “disuse”: It can readily be observed that exercise increases the size of muscles; that by steady application the capacity for thinking can be developed; that the oarsman’s constant use of his hands leads to the hardening and thickening of the cuticle; that the arm of the blacksmith and the legs of the mountaineer are much enlarged, etc.
When an organ is exercised properly, there is an increased blood supply to it, and, consequently, stimulated nutrition and growth in various parts, such as in the muscular, nervous or other tissues.
When an organ is disused there is diminished blood supply, and, consequently, diminished growth and functional capacity. In man it is known that certain activities, such as coal-heaving, shoemaking, etc., produce recognizable effects upon the muscular system, the skeleton, and other parts of the body.
The peculiar habits of a tribe, such as tree-climbing among those natives of the interior of New Guinea, who build their houses in the upper limbs of lofty trees, modify the body in ways that are readily recognizable.
After considering many facts in connection with the brains of rabbits, Darwin announced that this most complicated and important organ in an animal is subject to the law of decrease in size from disuse. We have very interesting illustrations of the effects of “use” and “disuse” in causing somatogenetic variations, in the differences between domestic ducks and the wild ones from which they have been undoubtedly derived. The wild duck, which must constantly be on the alert for enemies, and uses its wings so much more extensively and its legs comparatively less than the domestic duck, is a much more intelligent fowl than the stupid, well-protected domestic one. The wings of the wild duck are stronger and its legs shorter than those of the barnyard duck. It has been shown that in the wild duck the brain is nearly twice as heavy in proportion to the body as it is in the comparatively imbecile domestic duck.