l8 Darbishire, Laws of Heredity. 



Even if this is not a true history of the origin of the Law 

 of Contribution, the point about that Law which I wish to 

 enforce is that it is a physiological theory of heredity. It 

 is an attempt to account for the phenomena of heredity 

 by picturing the way in vvhich the characters of an 

 organism are represented in the germ cell which produces 

 it. And being a physiological Law, it is profoundly 

 different from a statistical one like Gallon's, which does 

 not pretend to account for anything, but is a generalization 

 about the relation between the aggregates of adults of 

 successive generations. 



In a word : a physiological Law deals with the indi- 

 viduals of each generation on both sides of the line AB in 

 the diagram : a statistical one, with the generation as a 

 whole on the upper side only. Another way of marking the 

 difference between physiological and statistical Laws of 

 heredity is to say that the former are explanatory while 

 the latter are descriptive. To which it will be immediately 

 objected that no Law ever explains anything. I am 

 perfectly aware of this, and of the fact, moreover, that no 

 theory does ; in fact that we cannot explain anything at 

 all in nature ; that all that we can do is to describe. But at 

 the same time it cannot be denied that there is all the 

 difference between attempting to account fora phenomenon 

 and contenting oneself with describing it : and one is, I 

 hold, perfectly logical in making this attempt to explain, 

 although one knows that however intimate a knowledge 

 of causation one has acquired one has done no more 

 than describe phenomena. One great difference between 

 these two things, the logical attempt to explain, and 

 satisfaction with mere description is that the method of the 

 former is experiment and that of the latter is observation. 

 Another difference is that it is only by attempting to 

 account for things that we have been enabled to get what 



