Introduction. 5 



Darwin's time. The changes brought about by it, the 

 Mutations, are phenomena as to the exact nature of 

 which we understand very httle so far. The best-known 

 examples of such Mutations are the so-called spontaneous 

 variations (the "single variations" of Darwin) by which 

 new and distinct varieties arise. They are also termed, 

 fitly enough, sports. In spite of the fact that they occur 

 fairly often, they are usually not noticed until the new 

 form has already appeared, when of course it is too late 

 to study the phenomenon of its origin experimentally. 

 These new forms can be sought for in cultivated species, 

 which are seldom of pure origin ; as well as in Nature. 

 But as yet we have no power of inducing them at will. 



It is my belief that all the simple characters of ani- 

 mals and plants arise in this way. . 



The methods of artificial selection correspond to these 

 two types of variability. Ordinary variation, which is 

 also known as individual, fluctuating or gradual varia- 

 tion, is always present ; and it can be described in terms 

 of perfectly definite laws which have now been fairly 

 completely formulated. It provides the breeder with 

 material for his improved races. On the other hand 

 he has to deal with Mutations which do not need repeated 

 selection but, at the most, must be kept free from ad- 

 mixture, and which almost always breed true from the 

 first. 



Under the general term variation, then, are included 

 two distinct phenomena : Mutability and fluctuation or 

 ordinary variation. The latter forms a suitable object 

 for statistical investigation. The epoch-making re- 

 searches of QuETELET and Galton on the anthropo- 

 logical side have raised this study to the position of an 

 independent science. Among biologists, Ludwig^ Wel- 



