us a true index of the mass of conjecture which was taking shape in his mind, but it shows us that he felt sure enough of the truth of his belief to allow a stronger tinge of evolution to appear in the second edition. He has mentioned in the Autobiography that it was not until he read Malthus that he got a clear view of the potency of natural selection. This was in 1838—a year after he finished the first edition (it was not published until 1839), and five years before the second edition was written (1845). Thus the turning-point in the formation of his theory took place between the writing of the two editions.
I will first give a few passages which are practically the same in the two editions, and which are, therefore, chiefly of interest as illustrating his frame of mind in 1837.
The case of the two species of Molothrus (1st edition page 61; 2nd edition page 53) must have been one of the earliest instances noticed by him of the existence of representative species—a phenomenon which we know ('Autobiography,') struck him deeply. The discussion on introduced animals (1st edition page 139; 2nd edition page 120) shows how much he was impressed by the complicated interdependence of the inhabitants of a given area.
An analogous point of view is given in the discussion (1st edition page 98; 2nd edition page 85) of the mistaken belief that large animals require, for their support, a luxuriant vegetation; the incorrectness of this view is illustrated by the comparison of the fauna of South Africa and South America, and the vegetation of the two continents. The interest of the discussion is that it shows clearly our a priori ignorance of the conditions of life suitable to any organism.
There is a passage which has been more than once quoted as bearing on the origin of his views. It is where he discusses the striking difference between the species of mice on the east and west of the Andes (1st