CHAPTER V. 



METHOD OF PROOF. 



In a matter of scientific inquiry one cannot go far wrong if one 

 follows the advice of Henri Poincare, who lays down certain prin- 

 ciples of method ; four of these are the following : — 



(1) The most interesting facts are those which can be used 



several times, those which have a chance of recurring. 



(2) The facts which have a chance of recurring are simple facts. 



(3) Method is the selection of facts, and accordingly our first 



care must be to devise a method. 



(4) We should look for the cases in which the rule established 



stands the best chance of being found fault with. 

 The groups of facts described in the succeeding chapters are 

 in agreement with these principles in the main, and are perhaps 

 like a dust heap for their intrinsic value. But one knows that 

 before now among a good deal of debris a rusty key has been found 

 which has opened a cabinet containing certain treasures, and in 

 the hands of someone else than the finder has produced useful 



results. 



The headings of the chapters describe the facts, and there is no 

 need to enumerate them here. The first and largest group is studied 

 according to a method which is in a measure applied to all the 

 others. Most of them are external or superficial j)henomena and 

 accordingly are open to others beside the expert for observation and 

 corroboration, or the reverse. The typical plan adopted is as 

 follows : a large number of related phenomena are chosen, and 

 the more prominent of these are observed and described. Keeping 

 in mind the two plain issues laid down, the origin of initial modifica- 

 tions and their transmission, I have selected the facts because, 

 especially such as those of the hair, they are very simple, of wide 

 distribu' ion in animals well known to us, such as the domestic horse 

 and man, and none are brought forward which any other observer 

 cannot study for himself if he has some anatomical and physiological 

 knowledge, some training and care in recording observations. In 

 most centres of population there are still left a good supply of 

 horses in streets and stables, of preserved specimens in museums 

 and living ones in zoological gardens, and of hairy young men who 



