406 Origin of Each Species Considered Separately. 



is a single one ; though its real nature is not yet apparent 

 to us. But this much is clear, that it is not, at least as a 

 rule, manifested as a single quality. And in this respect 

 a mutant differs from a variety, in which single qualities 

 like color, hairiness and so forth form the diagnostic 

 character. In the mutant this quality can only be mani- 

 fested in connection with the older characters of the 

 plant; and the total expression of this cause must there- 

 fore depend partly on the older characters and only 

 partly on the new factor itself. 



If we look at it in this way we can easily imagine 

 how a single internal change can bring about the ab- 

 normal development of the leaf parenchyma, of the pollen 

 cells of the anthers, of the petals, the fruits and the stig- 

 mas, and in this way produce the broad crumpled leaves, 

 the sterility of the pollen, the thickness of the buds and 

 the abnormal stigmas. 



This is of course only an idea. I mention it partly 

 to simplify the problem and partly because it may indi- 

 cate the lines along which the problem may be dealt with 

 empirically. In order to make my position clearer I will 

 now tentatively indicate the parallel which exists between 

 this idea and certain phenomena of parasitism. It is 

 generally admitted and, indeed, there can be little doubt 

 that the wonderfully definite and complicated structure 

 of the Cynipid-galls, with their nutrient tissue, their 

 stone-cells and their spongy tannin-containing outer par- 

 enchyma^ whose thickness is adapted to the length of 

 the ovipositors of parasites and Inquilinae, cannot be the 

 result of a single chemical stimulus. But it is an entirely 

 different matter with the cases of virescence since these 



^ M. W. Beyerinck, Bcobachtungen i'lher die crsten Entwiche- 

 lungsphascn ciniger Cynipidcn-Gallen. Verh. d. K. Akad. d. Wet., 

 Amsterdam, 1882. 



