Page:Popular Science Monthly Volume 63.djvu/504

500 over nation. Glancing backward, however, over that long and varied line of animal life shown to us in fossil forms—their history incontestably preserved for us direct from the hand of creation, we must readjust our vision, enlarge our horizon to grasp the significance of the origin, distribution, adaptation and survival of life. Life is here—life has been a feature of this old earth's history through countless ages. Whence came it, what have been the paths it has followed in its development and adaptation to the varied conditions of earth, sea, land and air? It would be presumption too gross to admit of toleration to assume to fully discuss so extensive a problem within the limits of a magazine article, but an effort will be made to point out in a rapid survey some of the factors that seem to have been effective in the peopling of the earth.

First, we should observe the conditions that have existed and that had to be met in the growth of organic beings—for we should not forget that life has had to adjust itself to conditions that existed prior to its appearance, since the conditions have not been modified to accommodate its needs.

Stretches of water and great reaches of land and the atmosphere furnish the basis upon which organisms must act, and either water or air, the medium that must serve them for many of their most vital functions. Glancing over the opportunities for survival of life in a delicate, simple condition, we can hardly fail to recognize the water as the most natural element for primal life-forms. Indeed, I think no naturalist will hesitate to consider water or, at least, an extremely moist location as the necessary condition for the beginning of life. Moreover, any serious consideration of the question must force the conviction that life-forms in other less favorable locations must have reached such location by gradual modification and adaptation. For instance, we can hardly conceive of the peopling of an arid desert-region with forms such as lizards, snakes, horned toads, scorpions, beetles, etc., except by the gradual encroachment upon desert area from adjacent territory by animals which were able to adapt themselves to desert conditions. Or, to put it in another form, the change to desert conditions in previously watered area must be accompanied by the driving out or extinction of all animals unable to adjust themselves to the new conditions. Assuming, then, a primary aquatic habitat for animal life, it becomes of interest to inquire both as to the most probable point of origin and as to the direction of adaptations.

While some naturalists argue for a pelagic origin of the simplest organisms, others hold to the idea of an origin near the shore, but in either case we have evident lines of travel from such a point to occupy the surrounding space. Thus from a near shore location life might work itself shoreward, adapting itself to the variable conditions of ebb