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or eye-teeth, fully developed grinders or molar teeth, and a power of jaw for which the civilized man of the present has no occasion. Just these peculiarities distinguished the skulls of some of the earliest known prehistoric races; and it seems but fair to assume that the man of our time, the lineal descendant of these savages, is losing in strength of teeth, largely because he is so entirely a cooking animal as he is.
Among the lower animals, similar causes have produced even more striking effects. the common hog, for instance, the effect of plenty of food, and a release from the dependence which the wild boar must place on ploughing up the ground for food, has been greatly to shorten the head, at the same time diminishing the strength of its bones and of the muscles of the neck, as may readily be gathered from the figure on the next page. So far has the shortening process gone, that, while the proportion of the length of the head to that of the body is as one to six in some of the comnion breeds of hogs (and much more than this in the wild boar), in the most improved breeds, the proportion is only one to eleven.
Mr. B. D. Walsh, one of the best known American entomologists, has shown that many kinds of insects vary much from the influence of the different kinds of plants on which they feed; and his investigations point strongly to the conclusion that even species are originated anew among insects by changes in their food.1
Color is one of the most variable of the characteristics of animals. As an example of the effect of food in producing color-changes, it may be stated that bullfinches and some other birds become black when they are fed on hemp-seed; while the common green parrot is, in South America, made to blossom out in red and yellow by being put upon a diet of fish-fat.
In such a case as that of the snails at Steinheim, so often cited, it is impossible to say whether it was a change of temperature, of food, of clearness in the water, or of all these, with still others besides, that produced the modifications of the snails, which are so manifest from layer to layer of the chalk-deposit in which they are found. But it is at any rate a very suggestive fact, that, whenever the character of the chalk-layers appears to have changed, the snails are found to have altered with it. No less
1 Darwin's Origin of Species, pp 38, 39.
remarkable is the fact, that young oysters transplanted from the north-west of Europe into the Mediterranean, at once begin to grow into the peculiar form, with diverging rays on the shells, characteristic of the Mediterranean oysters.
Allusion has been made to the effect of light, but it is not generally easy to trace this directly, except in the case of plants, where it is in part responsible for the extraordinary development of vegetation in tropical regions. Indirectly, however, its importance may be gathered from the appearance of such animals and plants as are partially or entirely withdrawn from its influ
Cave-fishes and cave-insects, for example, are, as is well known, frequently quite blind; and this blindness is generally attributed to the effects of disuse of the eyes, caused by the absence of light. Hardly less noticeable than the blindness of these animals is their transparent, blanched appearance, which is undoubtedly due to the partial or complete darkness in which they have lived. Cases are not wanting, again, of animals in which, from the effect of partial disuse, certain organs are found in a very im
| For an extended discussion of the bearings of cave-life on the theory of descent, see Professor A. S. Packard's Cave Fauna of N. A., Mem. Nat'l Acad. Sci., vol. iv., pp. 137–143.
perfect condition. It is to this cause that most naturalists would attribute the defective eyes of the mole; and Mr. Darwin cites the case of a burrowing South-American rodent, the tucotuco, in which he often found the eyes inflamed and upon the point of becoming useless, probably from the animal getting dirt into them while underground. But in this interesting case the result, after many generations, would probably be, that the animal (under the operation of a law which will be stated in the succeeding chapter) would at length lose its eyesight altogether.
It may be in some measure due to the absence of light, that horses kept for several years underground, in the Belgian coal mines, become covered with a soft coat of fur, like that of the mole.
Naturalists, everywhere, have been led by the stimulus imparted by the writings of the pioneers of organic evolution to bestow more and more attention upon the influence exerted by the surrounding conditions upon all living organisms. There seems to be at present a growing tendency to emphasize the importance of the changes in form and structure brought about by the direct action of the conditions of life upon the individual plant or animal, and to
1 A gnawing animal, like the squirrel, or the rat.