58 INTRODUCTION. 
If it be granted that the Darwinian theory seems sound in the 
main, though disputable and perhaps false in some of its parts,— 
it is necessarily granted also, that there is and can be no absolute 
difference between species and varieties; that they are groupings 
of individuals which differ in degree, but not in kind. Where the 
differences are so many, or so wide, or so slowly convertible, that 
numerous generations must have been required for diverging one 
from the other,—or, it may be added, would be requived for con- 
verging one into or with the other,—then, the two groups of 
individuals thus decidedly differing, may be accepted as two 
species. On the contrary, if their differences are of a slighter 
character or constancy, of so small a degree that the production of 
one of them from the other seems probable on analogy, although 
not actually observed,—then they are more properly to be 
accounted varieties, or type form and variety, of a single species. 
When the much discussed question is re-viewed under this 
theoretic light, the same conclusion is at length come to, as was 
almost unavoidably reached from a consideration of the wide dis- 
crepancies of opinion and practice among technical botanists; who 
are so much at variance among themselves in deciding which are 
species, and which are yarieties only, among their described 
plants. Through Darwinism, or some modification of Darwinism, 
we come back again to the former alternating definition, and may 
repeat that “Species are wider varieties; varieties are closer 
species.” Or, nearly the same idea may be expressed in another 
form by saying, ‘ Varieties are infantile species, Species are adult 
varieties.’ There are many stages between infancy and ado- 
lescence ; and botanists will continue to differ as to the stage of 
development where the one becomes the other. 
Perhaps it may be thought that too many pages have been 
given to these remarks on the grades of species, and on the 
relation of varieties to species, which is but carrying the gradation 
into further detail. Certainly the remarks have run out to con- 
siderable length ; but a writer on phyto-geography, during present 
unsettled opinions, ought to let his readers have a clue to his 
own views, and especially to make them aware of the difficulties 
