An Introduction to a Biology 



diagram ; a statistical one, with the generation as a whole 

 on the upper side only. Another way of marking the differ- 

 ence 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 for a 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 knowledge we have of the causa- 

 tion of natural phenomena and so to obtain what control 

 we have of the operation of natural processes. Just be- 

 cause we know that explanation is after all only description, 

 it does not follow that we should abandon the attempt to 

 account for things. 



We are now in a position to classify Laws of heredity 

 under these two headings : 



Physiological. Statistical. 



(a) Mendel's Law. (a) Galton's Law. 



(b) The Law of Contribution. (b) Pearson's Law. 



Now inasmuch as of physiological Laws b has been shown 

 to be invalid, and of statistical ones a has been shown to 

 be less comprehensive than b, the discussion of the mutual 

 relation of physiological to statistical Laws of heredity 



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