THE CAUSE OF VARIATION 93 



of its division. There seems to be no escape from the con- 

 clusion from which Darwin unaccountably shrank, that there 

 exists in all living things an innate tendency to vary independent 

 of external conditions, and in his own language "inevitably 

 contingent on reproduction." " Nevertheless," he says, " when 

 we reflect on the individual differences between organic beings 

 in a state of nature, as shown by every wild animal knowing 

 its mate, and when we reflect on the infinite diversity of the 

 many varieties of our domesticated productions, we may well 

 be inclined to exclaim, though falsely as I believe, that variability 

 must be looked at as an ultimate fact necessarily contingent 

 on reproduction." But while he produces abundant justification 

 for such an exclamation, I can find no grounds for the reserva- 

 tion, " falsely as I believe." The whole argument sadly lacks 

 the beautiful obviousness and simplicity which so distinguish 

 the theory of natural selection. It has a strong flavour of post 

 hoc propter hoc. In wild creatures we do not usually know what 

 variations may occur. When they are domesticated, any 

 variation, or at all events any marked variation, is apt to be 

 noticed, and perhaps seized as a peg on which to hang a 

 new variety. When it occurs, anything convenient is set 

 down as the cause ; in imported productions change of climate, 

 in home-grown creatures change of soil, change or excess of 

 food, breeding out or breeding in, anything in fact but an 

 innate power to vary. Such vague and contradictory sug- 

 gestions give no rest or satisfaction to the mind, and incline 

 the student to resort to a cause which, if true, is necessary, 

 general, and in accordance with forces which are manifested, 

 not only in the variations of organisms, but throughout nature, 

 in a process of universal and unceasing differentiation. Such 

 a cause appears to me to exist in the differential division of 

 the cell. 



In this passage Darwin finds the cause of variation in 

 change of the environment, nutrition being included in that 

 term. Weismann, who formerly held variation in the metazoa 

 to be effects of sex, and in monads to be due to the action 

 of the environment, afterwards admitted the force of the argu- 

 ments by which sex is shown to act in the opposite direction, 

 as a cause not of unlikeness but of likeness among the indi- 

 viduals of a species, and ceased to regard it as " the real root 

 of variation itself." This phrase and the following passages, 



