loo HEREDITY AND VARIATION 



points out, we are ignorant in regard to the conditions in which 

 mutations occur. The Mutation Theory does not as yet give 

 us a theory of mutations. 



§ 8. Causes of Variation. 



In regard to the causes of variation it is too soon to speak, 

 except in tentative whispers. What Darwin said must still 

 be said : " Our ignorance of the laws of variation is profound. 

 Not in one case out of a hundred can we pretend to assign any 

 reason why this or that part has varied." 



Variability. — The difficulty which every naturalist has felt 

 in trying to define the concepts of variability and variation is 

 due to the fact that living creatures are individualities — in some 

 degree, personalities. In the ocean of matter and energy or- 

 ganisms are, as it were, whirlpools, each one with a particular 

 character of its own. They are animate systems, each with a 

 unity or individuality which we cannot fully interpret. They 

 have the power — again an ultimate prerogative — of giving rise 

 to other whirlpools, to other animate systems, which tend to be 

 like themselves. But because each organism is a very complex 

 whirlpool in a very complex environment, and because a living 

 individuality cannot reproduce others without subtle molecular 

 manoeuvres which we know only in a far-off sort of way, one 

 individuality is very unlikely to reproduce an absolute facsimile 

 of itself. It is of the very essence of a living thing to change, and 

 an individuality cannot be halved. From this point of view, 

 variation is a primarily normal occurrence, and breeding true has 

 secondarily come about as the result of restriction. In short, 

 variability is a primeval character of organisms. We cannot 

 explain variability ; it is a datum in the world of life. We may, 

 however, try to show in certain cases how it operates and what 

 conditions help or hinder it. 



The unending problem of life is to establish some sort of modus 



