DISCONTINUOUS VARIATION. 65 



and the other smooth or glabrous. Intermediate forms 

 were also found, but these were scarce. Some of the 

 ripe seeds of these plants were obtained, and grown in 

 England. A portion of the seedlings were derived 

 from cross-fertilised seeds of known origin, and it was 

 found that though there was to a certain extent a blend- 

 ing of parental characters as regards hairiness and 

 smoothness in the offspring of plants of dissimilar types, 

 giving rise to intermediate forms, yet this intermediate 

 condition was found only quite exceptionally among 

 full-grown individuals. It was much more common in 

 the young plants, but as these grew older, their leaves 

 became smooth, and hence almost all the plants were 

 ultimately either hairy or glabrous; that is to say, they 

 varied discontinuously. 



Supposing a dimorphic condition is due to internal 

 causes, or to the fact that it is the nature of the plant 

 to vary in this way around two " positions of organic 

 stability," as Galton has termed them, rather than 

 around one such position, then it would seem almost 

 impossible to get further to the root of such causes. 

 Supposing, on the other hand, as is probably true in the 

 majority of instances, this dimorphic condition has been 

 derived from a previous monomorphic one, then we 

 may hold a more reasonable hope of being able to eluci- 

 date the cause or causes of this evolution from one con- 

 dition to another. The problem of the splitting up of 

 species was recognised by Darwin to be one of immense 

 importance, and he discussed it at some length in the 

 " Origin of Species." * The chief cause of divergence 

 of character he attributed to the circumstance that 

 *Ed. vi. p. 86. 



