VARIATION AND HEREDITY 



15 



been the origin of many new varieties of domestic 

 animals and plants, he favoured the view that to small 

 or continuous variations we must look for the chief 

 cause of evolution. But difficulties in that theory 

 soon appeared. A fully developed organ may prove 

 most useful, when in a rudimentary stage it would have 

 no effect on survival. Unless it arose per saltum^ 

 natural selection would never cause it to develop. 

 Again, a small variation would be bred out of the race, 

 unless the individuals possessing it became sterile when 

 mated with the older type, or refused so to mate. 



Such difficulties, together with a careful study of 

 sudden mutations by de Vries, Bateson, and others, 

 have led to a revision of the purely selectionist view. 

 Many biologists hold nowadays that discontinuous 

 variations, or " sports," have contributed extensively to 

 the separation of species. Such " sports " tend to 

 transmit their properties to their offspring ; they 

 certainly arise in some cases ; natural selection would 

 undoubtedly act on them in one way or the other as 

 they were useful or deleterious to their possessors ; 

 they can hardly help being a true cause of evolution. 

 Whether the small or continuous variations are also 

 effective remains an open question. 



Small variations may be divided into two kinds : 

 those which are innate ; those which are acquired. 

 Innate variations tend to be inherited according to 

 laws we shall study below. But much discussion has 

 arisen about the possibility of the inheritance of 

 acquired characters. 



The inheritance of such characters was generally 

 assumed, until Weismann called attention to the fact 



