PHYSICAL CONDITIONS AT BEGINNING OF LIFE 581 



ioo° C, the boiling-point of water, was probably but a few 

 thousand years ; and the drop from this to somewhere near the 

 present temperature possibly required only a few hundred 

 years. This would have been a period of great chemical 

 change ; and although it is perfectly conceivable that in this 

 period reactions may have taken place resulting in the basic 

 compounds of living substance, our knowledge of chemical 

 processes at high temperature is as yet too slight to permit 

 of any practical deductions or theories. 1 But if anything is 

 certain, it is that the quantity of carbon dioxide was never large, 

 and, as we shall see, the origin of life from a carbon dioxide 

 or carbonate basis is the simplest conceivable theory. 



On the meteoritic or planetesimal hypothesis, the only con- 

 ceivable conditions wherein a sufficient concentration could 

 occur of the substances for the organic or vital synthesis would 

 be in volcanic action. 



In other words, it seems probable, in the light of our present 

 knowledge, that our ideas as to the terrestrial Genesis of Life 

 will follow much the same course as that of all physical theories, 

 and notably those of geology. The beginnings will be the 

 most fantastic imaginings — as, for example, Kepler's supposition 

 that the earth is a vast breathing animal. These fancies of 

 intellectual childhood will in time be replaced by more philo- 

 sophical conceptions, reaching a period akin to the catastrophic 

 stage. This is represented by pictures of a boiling, sizzling 

 earth, with a continuous downpour of hot hydrochloric and 

 other acids, an atmosphere saturated with carbon dioxide and 

 under tremendous pressure, several hundred times that of the 

 present, as in the ideas of Sterry Hunt 2 and others. And finally 

 we shall come, as in all our scientific theories, to perceive that 

 change is slow, that the observable forces of the present day 

 are adequate for the explanation of events in the past, that more 

 or less what is, was. In brief, we should probably find, could 

 we recall it, that the primeval earth would not be so astonish- 

 ingly different from the earth as we know it now. 



While our ideas on the physical side have been, and are 

 still, undergoing this process of simplification, a like process 

 has been at work in the field of pure biology. We are now 

 fairly agreed that the myriad forms of life are due to a slow 



1 Clarke, I.e. p. 232. 



2 Chem. and Geo I. Essays, p. 35, 1874. 



