3 o8 SCIENCE PROGRESS 



Having stated my views with regard to the nature of the 

 earliest forms of life, we may now consider briefly the possible 

 origin of the primitive living organism. Here, however, we find 

 ourselves at once on uncertain ground, where the obscurity 

 which the present state of our knowledge cannot penetrate 

 makes it as easy to frame vague hypotheses and speculations as 

 it is difficult to find any solid basis upon which to take a firm 

 stand. Almost all that we can do with any profit is to limit to a 

 certain extent the possibilities of the problem by means of certain 

 propositions, for the most part of a negative order and therefore 

 not a very sure foundation for deductions. Thus from the 

 conclusions of astronomers and physicists with regard to the 

 past history of our solar system, it appears highly probable that 

 the terrestrial globe was once an incandescent mass at a tempera- 

 ture very much higher than that at which life of any kind can 

 exist ; consequently there must have been a period at which 

 there could have been no living things on the earth. On the 

 other hand, from all scientific experience it appears to be an 

 established truth, so far as a negative proposition can ever be 

 established, that living things at the present time are produced 

 only as the offspring of pre-existing living things and do not 

 arise de novo. From these two propositions taken together, it 

 may be concluded that there must have been a period or epoch 

 of time during which terrestrial life originated. Then there 

 remain two possibilities, the first that life took origin on the 

 earth itself, the second that it was brought to our planet in some 

 way from without. 



Biologists, I think, have generally been inclined to favour the 

 first of these views, namely, that terrestrial life was generated on 

 the earth itself; physicists, on the other hand (using the word 

 " physicist " in its widest sense), have been more prone to take the 

 view that living particles were wafted on to our planet from 

 interstellar space. It seems to me that the final word in the 

 matter will lie with the chemists. The main difficulties of the 



presented by the most specialised types of living beings. In my opinion there is 

 only one thing common to all forms of life, to the bacterium as well as to man or 

 to the oak-tree, that is the chromatin-substance. Further, I do not think that 

 the evolution of living beings can be considered profitably from the highest forms 

 downwards and backwards ; it is best worked, in my opinion, in the direction it 

 must have taken — that is to say, from the simplest and apparently earliest forms of 

 life onwards to the more complex and specialised. 



