Preface to the First Volume. ix 



sports are the best known instances, constitute a distinct 

 province in the study of variabiHty. They occur without 

 transitional gradations and are rare ; whilst ordinary varia- 

 tions are continuous and always present. 



The whole subject of variability, therefore, falls into 

 two sections, one of which includes the ever present, indi- 

 vidual or fluctuating variability, whilst the other embraces 

 mutability. The former phenomena conform to the well- 

 known laws of probability and are determined by general 

 nutritional conditions ; they also afford the material for the 

 production of many of the so-called improved races of agri- 

 culture. 



Mutations give rise not only to species but also to varie- 

 ties ; and, as has been recognized for a long time, they play 

 an all-important role in horticulture. An exhaustive com- 

 parative and experimental study of horticultural varieties 

 is an indispensable preliminary to a complete treatment of 

 the problem of the origin of new forms. It will be given in 

 the second volume. 



The generalizations here outlined apply obviously to 

 animals as well as to plants. Though as a botanist I have 

 confined my attention to the latter, I am convinced that 

 my results will be confirmed in the realm of the animal 

 kingdom. Again a proper distinction between variability 

 and mutability is of the greatest importance from the point 

 of view of the application of the results of biological investi- 

 gation to the solution of sociological problems. For, the 

 question of the origin of species has really very little to do 

 with these highly important problems ; whilst that of fiuc- 

 tuating variability is intimately and fundamentally bound 

 up with it. 



The contrast between these two groups of phenomena, 

 variability (in the strict sense) and mutability, becomes ob- 

 vious when we imagine that the properties of organisms 

 are built up of perfectly distinct and independent units. 

 The origin of a new unit is a mutation ; but the new unit 



