VARIATION AND NATURAL 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 was 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 purposes 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 wouldlhave been obliterated. 

 Thus he shows 'that : 



