How do these variations arise? Not by chance, of course (for in the strict scientific sense nothing on earth can be considered as really fortuitous), but as the outcome for the most part of very minute organic causes, whose particular action it is impossible for us to predict with our present knowledge. Some physical cause in each case there must necessarily be; and indeed it is often possible to show that certain changes of condition in the parent do result in variations in the offspring, though what special direction the variation will take can never be foretold with any accuracy. In short, our ignorance of the laws of variation is profound, but our knowledge of the fact is clear and certain. The fact alone is essential to the principle of natural selection; the cause, though in itself an interesting subject of inquiry, may be safely laid aside for the present as comparatively unimportant. What we have actually given to us in the concrete universe is, organisms varying perpetually in minute points, and a rapid rate of increase causing every minute point of advantage to be exceptionally favoured in the struggle for existence. But Darwin is remarkable among all broachers of new theories for the extraordinary candour and openness of his method. He acknowledged beforehand all the difficulties in the way of his theory, and though he himself confessed that some of them were serious (a statement which subsequent research has often rendered unnecessary), he met many of them with cogent arguments by anticipation, and demolished objections before they could even be raised against him by hostile critics. Of these objections, only two need here be mentioned. The first is the question, why is not all nature even now a confused mass of transitional forms? Why do genera and species exist as we see them at present in broad distinction one from the other? To this Darwin answers rightly that, where the process of species-making is still going on, we do actually find fine gradations and transitional forms existing between genera, varieties, and species.[3] But, furthermore, as natural selection acts solely by the preservation of useful modifications, each better-adapted new form will always tend in a fully stocked country to oust and exterminate its own unimproved parent type, as well as all other competing but less perfect varieties. Thus natural selection and extinction of intermediates go for ever hand in hand. The more perfect the new variety, the more absolutely will it kill off the intermediate forms. The second great difficulty lies in the question of the origin of instinct, which, as Darwin shows, by careful inductive instances, may have arisen by the slow and gradual accumulation of numerous slight yet profitable variations.