the general temperature of the earth's surface rose or fell 40° (a small amount relatively), the whole course of life would be changed, even perchance to extinction. The record of the fossiliferous rocks shows us that for countless millions of years a large portion of the earth's surface has had a temperature much the same as it now has; it is even probable that the surface temperature never greatly exceeded 40° C, though the interior was, and is, very hot.
Water plays an indispensable part in both the environment and the internal chemistry of life. It forms more than half the weight of most living things; and all the actively living parts of animals and plants (e. g., the nuclei and protoplasm of cells) consist of water holding the other ingredients in solution or suspension.
Every one of the conditions above mentioned (supply of energy, particular elements, range of temperature, abundance of water) is essential to life—i. e., such life as is known to us; and it is difficult to avoid the conclusion that this life is really the outcome of the conditions existing on our earth, and that only in worlds with identical conditions can identical life exist.
It is quite odd how, in spite of the advance of biological science and the acceptance of the principles of evolution, the notion still prevails that life in other worlds is similar to that of our earth. We find astronomers searching for the absorption bands of chlorophyll in the spectrum of Mars; marks on planets are described as probable vegetation; some worlds are supposed to be uninhabitable because they have no atmosphere, others because the temperature is too high for the existence of 'protoplasm.' All this indicates a very contracted view of the nature of life. Chlorophyll, respiration, vegetable, animal and protoplasm are earthly phenomena which may exist nowhere else: their place may be taken in other worlds by other phenomena no less wonderful. What we know of terrestrial life gives us reason to think that the same principles which produce life under earthly conditions may produce life of a different type under different conditions; e. g., where the temperature is different, and a different set of elements are available.
It must be freely admitted that we do not know what elements could take the place of nitrogen, carbon, etc., under conditions differing from those on our earth. All speculations concerning this question have been based on misconceptions of the functions of the elements in life. We may only venture so far as to say that certain elements suggest possibilities of energy traffic by reason of the varied character of their compounds: such are phosphorus, sulphur, iodine and iron. Other elements, such as aluminium and silicon, are remarkable for the monotony of their known chemical actions....
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In order that any world may support life comparable with the life
