VAEIATION AND NATUEAL SELECTION 505 
Now the aspect of the problem had been changed by Darwin 
and Wallace ; writers must be freer to adopt such a theory 
as may best harmonise with the facts adduced by their own 
experience. For they had greatly influenced the theoretical 
questions as to the origin and ultimate permanence of species, 
though he still held, as then, that consideration of existing 
species alone was insufficient to decide as to ancestry or origin- 
ally created types. The answer w^as to be drawn from the 
patient study of variation with its causes and checks of the 
distribution over the globe of living and fossil forms, leading to 
survival and extinction. 
In the New Zealand Flora his experience had already led 
him to insist on the variability of plants, far greater than was 
generally recognised, and he had indicated that it is to the 
extinction of intermediate species and genera that we are 
indebted for our means of resolving plants into definable 
genera and species, a position generally accepted by believers 
in the permanency of species. He was now moved to show how 
far we may extend this view to the limitation of species them- 
selves by the elimination of their varieties through natural 
causes. 
Still, though it is only an arbitrary line, a question of 
degree, that separates genera and species and varieties, he 
continues to use the term species as the coin of science, which 
for practical pm^poses of description passes current among 
believers in mutability and permanence alike. 
The moment had come to write those general essays on 
variation and distribution in plants which Darwin had often 
urged him to write, reviewing in the light of all the new evi- 
dence those questions which, on the botanical side, he had made 
his special study for so many years. The conclusions which 
emerge as to the extent of variability and the balance between 
the forces of nature which make for change and for permanence 
immediately arrested the attention of his fellow workers, who 
were often met by statements that variation on a large scale 
did not exist, or that if it did exist, all specific distinctions as 
we know them wouldThave been obliterated. 
Thus he shows that : 
