1 18 
EVOLUTION 
term is as yet impossible. Most of the great systematists, 
e.g. Sir Joseph Hooker, use rather comprehensive species, 
often including many varieties ; such species are now often 
called “Linnean species,” in contradistinction to the ele- 
mentary or “Jordan’s species,” which are considered as 
varieties by Hooker. Botanists who work mainly at the 
flora of a limited region, or at single groups of plants, usually 
tend to make small species. The balance of opinion at 
present favours the use of large species, and this course is 
followed in the present work, and in the vast majority of 
botanical books. The student will find it of interest in this 
connection to study such genera as Rubus, Salix, Hieracium, 
&c. with the different British Floras of Hooker, Bentham, 
Babington, &c. 
Two nearly allied species will have many characters in 
common derived from the common ancestor, but will show 
more differences, and those more clearly marked, than 
varieties. The latter when crossed usually yield fertile 
offspring, whereas the offspring ( hybrids ) of two species are 
usually comparatively sterile (but cf. the case of heterostyled 
flowers, p. 95). The student should work at the determina- 
tion of species until familiar with the kind of characters that 
mark specific distinction ; he will note that they are chiefly 
characters of the vegetative organs, and minor characters of 
flowers, fruits, &c. 
The original species A has thus, after a considerable 
lapse of time, given rise to four new species, T, U, V, W, by 
which it is now represented on the earth. Now consider the 
case of B, a species nearly allied to A and represented by 
few individuals. The chance of a favourable variation 
appearing in a number of the offspring is smaller; let us 
suppose that none appears, and that B continues unchanged, 
until the general ecological conditions change, or an un- 
favourable variation occurs, or other forms come into closer 
competition with it. It may then gradually dwindle in 
numbers and finally die out. Extinction of species is a 
common occurrence, as geology teaches. Now take the 
case of C, also a species of few individuals. This we may 
suppose not to become extinct during the period under 
consideration, because it has had a favourable situation, 
