100 TEE AMERICAN NATURALIST [Vol. LVI 



This is a subject that I have elsewhere discussed at 

 length. Hence it will perhaps suffice briefly to recapitu- 

 late a few of the more striking facts. Because of its 

 peculiarities as a solvent, as an ionizing medium, etc., 

 water makes possible the formation of an almost indef- 

 initely greater variety of physico-chemical systems than 

 does any other substance. On account of its high latent 

 heat of vaporization, its high specific heat, its high sur- 

 face tension, and the peculiarities of carbonic acid, such 

 systems often possess a very remarkable stability. The 

 elements hydrogen, carbon, and oxygen, of which water 

 and carbon dioxide are composed, seem to be unique in 

 the number and variety of the substances which they can 

 form. In particular, the production of sugar from water 

 and carbon dioxide fixes a very great amount of energy 

 and leads directly to the greatest variety of chemical sub- 

 stances and reactions which are known to occur as the 

 result of one chemical process. Finally, water and car- 

 bon dioxide are the two substances which are everywhere 

 available. 



Anything so complex, so stable and yet so variable, so 

 widespread and so active as life can only occur when a 

 great variety of conditions are fulfilled. In other words, 

 the physico-chemical systems of the organism, in order 

 that life shall be capable of its evolution, must possess 

 altogether exceptional characteristics, which appear to 

 be quite impossible unless water and carbonic acid, and 

 compounds of the three elements, hydrogen, carbon, and 

 oxygen, and no doubt also of nitrogen, are at the basis of 

 them. These substances possess a set of properties each 

 one of which by itself and also in cooperation with the 

 others is necessary for the highest physico-chemical com- 

 plexity and variability. So far as w^e know, no other ele- 

 ments or compounds possess another set of properties 

 which permit similar physico-chemical complexity or 

 variability. 



It is, I believe, for this reason that life has always op- 

 erated on the same basis. 



