Page:Popular Science Monthly Volume 64.djvu/505

Rh traversing the universe, we should find beings akin to ourselves in many a remote corner of space.

On the earth, indeed, life exists under conditions which may be widely departed from in many other planets. Here the extreme range of favoring temperature is that between the freezing and the boiling points of water, the practical range being much smaller. Special conditions of surface material and formation, atmosphere, chemical action, etc., are also necessary. It is far from certain that the same conditions are necessary everywhere. Life may perhaps flourish on other planets under quite different conditions of temperature, gravitation and chemical action. It is true that, if all the spheres of space are made up of essentially the same chemical elements, as spectrum analysis seems to show, the range of life conditions can not greatly vary. Yet if the more abundant and active elements in any sphere differ from those of the earth, the consequent life conditions might vary accordingly and life exist under relations of temperature and chemical action unknown to us. The one thing essential, in every case, is an environment favoring organic chemism.

All this, however, is a side issue. It has no necessary bearing upon the question of animal form. If human beings could exist on some planets at 1000° instead of 100° F., and be made up of a protoplasm of quite different chemical composition, their forms and modes of action might still be closely the same. For the external forms of animals are due to physical, not to chemical, conditions. They are mainly results of the struggle for existence, and the effort to gain the most effective formation for the incessant battle of life. This must go on wherever life appears and develops, wherever the temperature or the active chemical elements may be. Much the same may be said of internal development. It seems to us that in any advanced stage of life the energy of animal motion must be a consequence of chemical change, due to something equivalent to oxidation of the tissues. There must also be an efficient agency for the supply of fresh nutriment to the wasting tissues, nerves for sensation and muscles for action, excretive and reproductive organs, etc., in short, organic conditions analogous to those which exist in our own bodies.

In truth, the minuteness of the earth as a planet, and the seeming insignificance of its life story as compared with that of all spheres and all periods, are apt to give us a false impression of the real significance of the development of life upon our place of abode. Though the process of organic evolution here may seem to us a minor one, a review of its history will serve to show that it has been a major one. And its final outcome in man can hardly be looked upon as a fortuitous result, but seems rather the inevitable consequence of an innumerable series of experimental variations. The life period upon the earth has