146 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



would also have come into existence on the other similar worlds? 

 There are many who take the view that life on the Earth is the result 

 of a special act of creation and that only by a similar special act of 

 creation could life come into being on any other world. If we accept 

 this view, no further discussion is possible. On the other hand, we 

 may take what seems to me the more rational view, that life somehow 

 came into existence on the Earth because conditions were favorable 

 for it ; and that if elsewhere in the universe the conditions are favor- 

 able, there life will be found either now or in the future. 



COMPOSITION OF THE LIVING CELL 



Suppose, in the second place, we find that there are worlds on which 

 conditions are so different from those with which we are familiar that 

 no form of life to be found on the Earth could exist under them. Will 

 it be legitimate to assume that such worlds must be devoid of life? It 

 might be supposed that the forms of life that exist on the Earth have 

 developed, through the slow process of evolution, in adaptation to those 

 conditions and that on another world, where conditions are entirely 

 different, types of life bearing no resemblance to any terrestrial forms 

 might have developed. Living cells of both plant and animal life 

 on the Earth, though they show a diversity in structure, all consist 

 primarily of the elements carbon, oxygen, hydrogen, and nitrogen; 

 sulphur, phosphorus, potash, soda, lime, and other substances may 

 be present in smaller quantities. It is conceivable that on another 

 world the living cells might have a different composition. They might, 

 for instance, contain silicon in place of carbon and, as a result of this 

 difference, they might be able to exist at temperatures high enough to 

 destroy all terrestrial forms of life. I think that such a possibility 

 must be ruled out. The same elements, subject to the same chemical 

 laws, are to be found throughout the universe. Moreover, there is no 

 reason why cells of such different composition should not exist on the 

 Earth, yet we do not find them. 



"COMPLEX AND FRAGILE" 



Assuming, then, that there must be a general uniformity in composi- 

 tion of the elementary cells, wherever life may exist, we have some 

 justification for concluding that certain essential conditions are neces- 

 sary for life of any sort to be possible. The molecular structure of all 

 living cells is both complex and fragile. A molecule of the protein 

 called albumen, for instance, is built up of 72 atoms of carbon, 112 

 atoms of hydrogen, 18 atoms of nitrogen, 12 atoms of oxygen and 1 atom 

 of sulphur. Such complex structures are readily broken up. High 

 temperatures are very effective in breaking up these complicated mole- 

 cules. All forms of life that we know are very sensitive to heat, and the 



