82 INHERITANCE IN ANIMALS 



exact prediction of future experience, than others. This 

 difference in the degree of accuracy with which different 

 kinds of experience can be described has a really im- 

 portant influence on the methods employed in different 

 sciences ; and before I can attempt to show you how the 

 methods employed in the study of organic Inheritance 

 differ from those employed by an astronomer or a physicist, 

 I must ask you to consider for a moment what experi- 

 mental exactness means, and how nearly it is attainable 

 by any one. 



The only way in which I can show you what absolute 

 exactness means, is by reference to a science which does 

 not appeal directly to experience at all the science of 

 Pure Mathematics. There is a certain geometrical state- 

 ment, familiar to all of you, to the effect that the three 

 angles of a plane triangle, taken together, are equal to 

 two right-angles. I dare not guess, nowadays, how you 

 convinced yourselves that this statement is true; but I 

 want you to think of the way in which Euclid shows you 

 that you must believe it. He first gives you definitions of 

 straight lines, planes, and right-angles. He does not tell 

 you to get a plane and to draw straight lines and right- 

 angles on it for yourselves, because you cannot draw the 

 things he defines, and if you had drawn them you could 

 not see them. But Euclid says that so long as he is speak- 

 ing to you, his words will have the meaning given in his de- 

 finitions ; and although these definitions describe nothing 

 within the range of your actual experience, you can follow 

 their meaning sufficiently well to use them as instruments 

 of thought, just as you can think of a mermaid, or a 

 centaur, though you have never seen either. Well, the 

 next thing Euclid does is to tell you a few truths so 

 simple and so obvious that the moment they are 

 uttered you recognize their truth, and agree that they are 



