Page:Popular Science Monthly Volume 63.djvu/519

Rh starting on a downward path to extinction or an upward path to domination, a wanton sacrifice of rich legacies of structure or improvement of such as may be possessed?

While adaptations and progressive modifications have been the rule, there are a few striking exceptions, and we can not assert that preservation of a particular plane of development is impossible. Picture the little lamp shell, Lingula, living on unaltered from age to age, its fossils being found away back in the earliest Paleozoic time and in subsequent ages up to the present day—so much for staying at home and attending strictly to its own business. True no change—no progress—but as an example of the staying qualities nothing can be more striking.

Concluding, then, we may look in wide view at life as originating in most favorable conditions of moisture and temperature; as pushing out to occupy all favorable environments, and then, from the congestion of life in such places, as pushing out to extremes in various directions and, ultimately, by a process inherent in life itself, constantly but unconsciously striving to occupy every available niche having the remotest possible opportunities for the support of organic beings. The animal choosing its environment and the environment reacting to modify the structure of the animal.

But this pushing has been along different lines and some of these involve no such radical change of form or habit as to restrict the animal to a special environment. In such broad highways progressive evolution is still possible, and we may expect future modification, advancement, adaptation, the height to which advancement is possible depending on how fully the animal may preserve its general varied structure while reaching such perfection of organs as to enable it to dominate the forms with which it must compete for mastery.

Where the road narrows and the animal in traversing it is obliged to sacrifice some portion of its structure and to adopt some restriction of habit the result is a limitation which must ultimately mean a bar to all progressive evolution in the acceptance of a particular limited sphere within which it may survive, but to leave which means extinction. Adaptation to sedentary life, parasitism, desert, cave, deep sea, polar frigidity or extreme heat is to shut the door of progress and give over to mere survival.

We may be tempted to moralize a little, for a moment's thought assures us that man himself is, like other animals, subject to these inevitable laws, and that retrogression, degeneration, decay and extermination are as possible to him or to certain of his races as to any other form of life, but this subject widens into the great new field of sociology—one of the latest, richest and most important of the branches of biology.