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believe in the inheritance by the offspring of changes thus brought about in the parent.1 A good deal of evidence has been accumulated tending to show that this process of variation under the influence of some pressure or stimulus arising from the environment, followed by inheritance of the new traits thus acquired, has gone on, frequently with extreme rapidity, throughout geological time, and is still active in modifying the structure and functions of living beings. At the first glance it may appear uncertain whether any such law of inheritance exists. In the human family, for example, it is evident enough that the child of the expert gymnast is not nearly always possessed of exceptional muscular development, that the sou of the accountant is not necessarily born with an unusual power to master figures, and that a lifetime spent in acquiring the trained hand of the sculptor may have gained absolutely nothing transmissible to his children. But, in the long run, it appears that there is a tendency to the transmission of acquired characteristics, and not only this, but to their inheritance by

1 For a detailed account, see Karl Semper's Animal Life as Affected by the Natural Conditions of Existence. Int. Sci. Series, Appleton, 1881. See, also, National Acad. of Sci., vol. iv., Packard's Cave Fauna of N. A.

the younger generation at an earlier age than that at which the parent gained these peculiarities, whether of structure or habit. This law of concentration, due to the action upon the organism of disturbing forces outside of itself, greatly intensifies the rapidity and effectiveness of modifications arising from the relations of living beings to their environment.

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In connection with this subject a word must be said in regard to a curious fact; namely, that some animals may breed either in a fully devel

1 "It is interesting to observe that the power of descrying objects at vast distances appears to be hereditarily possessed by two races of men, the Mongols of Northern Asia, and the Hottentots of Southern Africa, both of which habitually dwell on vast plains that seem to stretch without limit in every direction. It seems probable that this power was in the first instance acquired by habit in every case: and that, as frequently happens with acquired peculiarities which are kept up by constant use in successive generations, it has become fixedly hereditary." Principles of Human Physiology, p. 779, W. B. Carpenter, Am. ed., H. C. Lea, 1876. See, also, Princ. Comp. Physiol., § 620.

oped or in an imperfect and widely different condition. The Mexican axolotl is a striking instance of the kind. It is a tadpole-like creature of considerable size, which lives in the water, breathes by gills, and is reproduced from eggs.

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In its native country, Mexico, this animal is not known to change its form, but hatches from the egg into a minute object much like a young tadpole, and gradually grows to the form and proportions shown in the figure above. Other species much like the axolotl occur in high regions, as in Wyoming, seven thousand feet above sea-level.

Now, in 1867, the astonishing fact was observed at the Jardin des Plantes, in Paris, that

some of these animals cast their skins after crawling out of the water, and began a new existence in the shape of a common salamander (Amblystoma).

How different-looking the salamander is from the axolotl the figures above given well illustrate. And the real difference between the animals is no less than the apparent difference: it may fairly be compared to that between a water-snail (which breathes by means of gills, and would soon die in the air) and a land-snail, which has a simple organ answering the purpose of a lung; so that the animal, if placed in water, would drown. But this astonishing change from axolotl to salamander, is accomplished in from fourteen to sixteen days, and, it seems, may always be brought about in healthy specimens by placing them in shallow water, and gradually diminishing the supply. The salamanders produced in the way just described lay eggs, which hatch into tadpole-like larvæ that soon grow into salamanders.

Now, since the axolotl-descended salamanders are of precisely the same species with other salamanders found in the western United States, it seems certain that these wild individuals are

1 Wilson, Chapters on Evolution, pp. 242-244.

descended from axolotls or the latter from these. If this change was due, as seems entirely possible, to decided alterations in the climate of the region inhabited by these animals, or to a change in their habits of life from more to less aquatic, or the reverse, we have here a striking instance of the modifying effect of a single element in the environment; namely, the presence or absence of water on animal life.

Reference must here be made to a general law which is further considered in Chapter VI.; namely, that during the life-history of every animal or plant it usually passes through stages in which it resembles individuals of other groups, often most widely different from the mature condition of the organism in question. Professor Cope cites the following examples of such occurrences in closely related groups:

All young deer are spotted, and some species of deer, found in eastern and southern Asia, are spotted in adult life. All deer have spike-shaped horns in their second year, and some never develop any more complex antlers. All larval salamanders are of uniform color, often olive, and some species retain the olive color throughout life. All infants have the calf of the leg and the bridge of the nose but slightly developed, and many African savages carry these

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