Page:Popular Science Monthly Volume 63.djvu/505

Rh and flow of water and exposing itself to such variations of conditions as might lead to fresh-water existence—to life on the land and thence to air, and, on the other hand, a dispersal on the surface to pelagic life. Again from shore-life or the original near-shore location, there might be a contingent working into deeper seas, and still further to occupation of abyssal depths. Transfers from surface to sea bottom, and the reverse, seem probable, in fact certain, for some groups and movements of terrestrial groups of animals into water must certainly have taken place. The peopling of all habitable corners of the earth then has been a process of continual pushing out from original centers, a constant, if unconscious, effort of animal life as a whole to occupy all available space, to crowd the energy of vital force to the end of every open channel, to follow every thoroughfare and explore every by-path that might lead to nook or corner in the universe that could give support in any fashion. But seldom has any form of life started alone on its travels, and hence the crowding for place, the 'pussy wants a corner' need, the eternal jostling to get and keep that corner and its opportunities, the 'struggle for existence' that has been the dominant principle of life from the dawn of its creation. Clearly those forms most successful in adaptation to new conditions must be those that win in the race and which soonest give rise to a higher and more complex form of existence. Whether life began in a single organism the parent form for all the mighty train that followed until now, or whether numerous organisms started independently, we can hardly doubt that all were equally simple, and similar courses of modification must have affected all. Moreover, in every case, we are warranted in assuming that for all higher types of animals there was a probably common ancestral form, and distribution over the earth must have been accomplished from an initial center by succeeding generations. Further, that for each particular subordinate group, family, genus or species that now has extended distribution, we must assume dispersal from the original home of the ancestral form.

First, then, we had only aquatic life, and this element may have been densely peopled before an effort was made to move ashore or to seek dry land. All geological evidence shows enormous development of aquatic life in early times, but obviously such forms were most likely to be preserved. Land life may have been forced by the drying up of stretches of water as well as voluntary migration. But what an important change that from aquatic to terrestrial life—from water-breathing to air-breathing! What possibilities of expansion, growth and occupation in the new, untrodden sphere, in the valleys and hills of earth and in the invigorating supply of air! Up this highway have come not a few of the great groups of animals. Some of the simplest protozoans even discovered the track, and worms of various kinds have crawled to