28 POPULAR SCIENCE MONTHLY. 



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 per- 

 chance to extinction. The record of the fossiliferous rocks shows us 

 that for countless millions of years a large portion of the earth's sur- 

 face 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 hold- 

 ing 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 con- 

 ditions 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 pre- 

 vails 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 con- 

 ditions may produce life of a different type under different conditions ; 

 e. g., where the temperature is different, and a different set of ele- 

 ments are available. 



It must be freely admitted that we do not know what elements 

 could take the place of nitrogen, carbon, etc., under conditions differ- 

 ing 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 sug- 

 gest 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. . , . 



In order that any world may support life comparable with the life 



