THE FACTS AND LAWS OF VARIATION 49 



order, implies a natural cause. In point of fact, 

 we all do make this assumption. We do not pray 

 that two days shall follow one another without an 

 intervening night, for the invariable sequence of 

 night and day has caused us to assume that it lies 

 within the sphere of natural causation. Similarly 

 the " Book of Common Prayer " still contains a form 

 of prayer for rain, meteorology not being sufficientl}' 

 advanced to enunciate such laws as would convince 

 unthinking men that the weather, also, is determined 

 by natural causation. 



Now, in the case of variation, it may similarly 

 be argued from the existence of the laws which the 

 Mendelians, the biometricians, and others have 

 discovered that this phenomenon, like all others, 

 is within the sphere of natural causation. It is, 

 then, submitted that a priori considerations and a 

 posteriori considerations (viz. the fact that it is pos- 

 sible to enunciate laws of variation), lead alike to the 

 same conclusion, that the causes of variation are to 

 be found in the natural order of thini^^s. So certain 

 is the evidence of these complementary lines of 

 reasoning that our faith in their common conclusion 

 is not shaken by the circumstance that, as we shall 

 see, the causes of variation are still somewhat 

 obscure. 



CHAPTER VI 

 THE ORIGIN OF VARIATIONS 



It is of the first importance that we should have 

 an understanding of the causes that produce 

 variation. It is so, not merely by reason of the 



