4 HUMAN BIOLOGY 



worlds, not accessible to present means of observation, may 

 exist within the known universe. 



THE CONDITIONS NECESSARY FOR LIFE 



These may be summarized as follows: 



(a) The Presence oj Those Chemical Elements which are 

 Essential Constituents of Living Matter. It was once uncertain 

 to what extent this condition might be fulfilled, and, indeed 

 whether elements unknown on earth might not exist, or 

 even predominate, in other worlds. The evidence of the 

 spectroscope has settled the matter. Lines which reveal 

 the presence of familiar terrestrial elements are found in the 

 spectra of the remotest observable nebulae, millions of 

 light-years distant. As science advances, one after another 

 of the groups of "unknown" lines which were supposed to 

 indicate the existence of strange elements have been iden- 

 tified as due to known elements under new conditions, 

 until hardly anything remains. The same elements, therefore, 

 are found everywhere in the universe. What is more, there 

 is good evidence that they are everywhere present in much 

 the same proportions, and that the well-known differences 

 between the spectra of the hotter and cooler stars arise not 

 from differences of composition, but from differences in the 

 physical conditions, which put sometimes certain elements, 

 and sometimes others, into a condition to absorb hues in the 

 rather limited spectral region which can be observed. 



Most of the important constituents of living organisms: 

 hydrogen, carbon, nitrogen, oxygen, magnesium, calcium, 

 iron, are shown by direct evidence to be widespread, if not 

 ubiquitous. For a few, such as phosphorus and chlorine, 

 evidence is lacking, on account of the absence of suitable 

 lines of these elements in the observable part of the spectrum; 

 but the general evidence for the uniformity of nature is so 

 impressive that there can be no doubt that these, too, are 

 to be found • wherever extensive aggregations of matter 

 exist. 



(b) The presence of certain compounds which are of especial 

 importance to life; and, most of all, of ivater and carbon 

 dioxide. The relation of these substances and their properties 

 to life has been so ably discussed by Professor Henderson 



