FACTORS OF NATURAL SELECTION 97 



not signify. So long as individual variation is present, 

 so long as it is hereditary, it does not signify how it is 

 produced. There are, indeed, many theories professing 

 to account for it ; but biologists are not generally agreed 

 as to the manner in which it is produced. But so long 

 as it is there, it is available, and Natural Selection can 

 make use of it. 



It is interesting to note that, when Newton discovered 

 the principle of Universal Gravitation, some people main- 

 tained that he had discovered nothing because he had 

 not explained what gravity itself was. Now, after over 

 two hundred years, we can safely assert that Universal 

 Gravitation stands out as one of the most triumphant 

 discoveries of the human intellect; and yet we, even 

 now, are just as much in the dark as to what gravitation 

 itself is as when Newton wrote. Exactly so it is with 

 regard to individual variation. So long as it is a fact 

 essential to organic nature, that one individual must be 

 different from another, and so long as these differences 

 are hereditary, so long may Natural Selection have abun- 

 dant material for its work, even though it does not itself 

 explain how that individual difference is produced. I am 

 very far from undervaluing the interest of such an explana- 

 tion; on the contrary, I maintain that it forms one of 

 the most interesting of biological problems now before the 

 scientific world, or likely to be before it for many a day. 



In fact, every successful attempt at scientific explana- 

 tion only interprets down to a certain level of causation ; 

 and this is just as true of Universal Gravitation and 

 Natural Selection as it is of smaller efforts. Down to 

 a certain level of causation, Natural Selection explains at 

 any rate some part of organic evolution. A more funda- 

 mental level would be to explain the factors upon which 

 Natural Selection itself depends ; but because we have not 

 yet reached that lower level, we have no reason for doubt- 

 ing, as some would believe, the complete efficiency, at 

 its own level, of the explanation we do happily possess. 



The theory which stands in contrast with Natural 

 Selection, and has been supported in the United States 

 more fully than in any other civilized country, with the 



