8 INTRODUCTION. 



Lamarck thought that the stability of a species is obtained as 

 soon as the reaction of a species to a changed environment is 

 definitely accomplished, as soon as the species has come into 

 a new state of equilibrium with its surroundings. 



Darwin rather denied the stability of species, and thought 

 that a moderate amount of variation is always present, and 

 Weismann thinks of the final stability and purity of a species 

 as the result of a long continued natural selection. 



De Vries holds, that the unknown causes for the abrupt var- 

 iation which produces a new species, imply a new stability. 

 Species spring into existence suddenly, and they are stable 

 from the very beginning of their existence. 



Our task is obviously a double one. We will have to treat of 

 variation, we will have to ask, how much the new facts which 

 Genetics has so far given us, have taught us about variation ; 

 whether we can distinguish between essentially different kinds 

 of variation, which of these kinds may be concerned with evo- 

 lution and which kinds are not, and we will have to show what 

 we now know about the causes of variation. On the other hand, 

 we will have for task, to examine how far the new facts have 

 taught us something concerning the way in which specific sta- 

 bility is attained. Both subjects merge one into the other; we 

 will see how far our answers to questions (as to the nature of 

 variation) help us to appreciation of the causes for specific 

 stability. 



