480 PROGRESS OF SCIENCE IN THE CENTURY. 



emphasising the so much neglected commonplace 

 that we cannot have scientific formulas mixed up 

 with any other interpretations in one sentence; and 

 that to place these other interpretations in opposition 

 to scientific formulae is to oppose incommensurables, 

 and to display an ignorance of what the aim of 

 science is. 



From the Fact to the Factors. So far then tho 

 formula, but let us pass to the more difficult ques- 

 tion of the factors. Evolution is a certain mode of 

 becoming, what are the operative conditions) Here 

 we pass from practical certainty to perplexing un- 

 certainty, as is so often the case when we pass from 

 the general to the particular, from abstract to con- 

 crete. 



Nature of Variations. The first great question is 

 aa to what may be called the raw materials of prog- 

 ress, the origin and nature of those variations or 

 organic changes on which the possibility of evolution 

 depends. 



Darwin started from the broad fact that variabil- 

 ity exists (illustrating it chiefly from domesticated 

 animals and cultivated plants) ; he postulated a crop 

 of organic changes, both of tares and wheat ; and he 

 pointed out how a process of ' singling ' and thinning, 

 sifting and winnowing would operate upon the ever- 

 growing, ceaselessly changing crop so that tho result 

 was progress. But all science begins with measure- 

 ment, and tho great step in advance that has been 

 made of recent years is in the dry and tedious, but 

 absolutely necessary, task of recording accurately 

 tho variations which do actually occur. 



Without being biologists, simply as clear think- 

 ers, we can see the unsatisfactoriness of the line of 

 argument which was until recently prevalent, that 



