ORIGIN OP SPECIES IN NATURE. 
201 
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 Royal 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. littorale, Link, 
Littoral; the passage to P. maritimum , L., sea-shores; var. 
agrestinum, Jord., the common robust field form ; arenastrum , 
Boreau, a sand-loving prostrate one; microspermum, Jord., a 
small fruited one; and rurivagum , Jord., a wayside one; sub¬ 
species, P. Roberti, 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 Student's Flora, p, 346. 
