ORIGIN OF SPECIES IN NATURE. 



ON THE ORIGIN OF SPECIES IN NATURE, AND 

 SUGGESTIONS FOR EXPERIMENTS TO INDUCE 

 VARIETIES TO ARISE UNDER CULTIVATION. 



By the Rev. Prof. G. Henslow, M.A., F.R.H.S., V.M.H. 



[Lecture delivered at the Gardens of the Eoyal Horticultural Society, 

 Chiswick, July 13, 1898.] 



Introduction. — What is a " Species " ? — It is a term used 

 by botanists to indicate a certain amount of differences between 

 one and another kind of the same genus. Hence a species is 

 known by a collection of morphological characters, presumed to 

 be constant, and taken from any or all parts of a plant. A 

 variety only differs from a species in having a less amount 

 of differences. There is no recognised standard as to the 

 amount or number of differences which separate varieties from 

 species ; hence systematic botanists have greatly differed in 

 their use of these terms. The point to be remembered is that 

 neither one nor the other is a fixed entity in nature ; but a 

 so-called variety or species can change its form under altered 

 circumstances ; and the direct cause of changes of form or of 

 variations of structure in plants is a change of environment. 



A good example of " forms " being assumed by a common 

 plant under different soils, &c. may be seen in the knot-grass 

 {Polygonum aviculare, L.). Thus Sir J. D. Hooker describes 

 the varieties: — " P. aviculare, L., proper ; P. littoralc, Link, 

 Littoral; the passage to P. maritimum, L., sea-shores; var. 

 agrestinum, Jord., the common robust field form ; arenastruvi, 

 Boreau, a sand-loving prostrate one; onicrosioermum, Jord., a 

 small fruited one; and rurivagum, Jord., a wayside one; sub- 

 species, P. Eoberti, Loisel, sandy shores." * 



In Nature, new varieties, Sir J. D. Hooker observes, are 

 mostly found on the confines of the geographical area of the 

 species : and when plants are grown for experiment in widely 

 different regions, they are generally found to lose their special 

 features, and to take on those of the plants among which they 

 now live ; so that lowland forms assume alpine or arctic features 



* Hooker's Sttident's Flora, p. 346. 



