F. To continue then with the cats. Famine comes and tests them, so to speak; the weaker, the less active, the less cunning, and the less enduring cats get killed off, and only the strongest and smartest cats survive; there will be no favouritism shown to animals in a state of Nature; they will be weighed in the balance, and the weight of a hair will sometimes decide whether they shall be found wanting or no. This being the case, the cats having been thus naturally culled and the stronger having been preserved, there will be a gradual tendency to improve manifested among the cats, even as among our own mobs of sheep careful culling tends to improve the flock. C. This, too, is obvious. F. Extend this to all animals and plants, and the same thing will hold good concerning them all. I shall now change the ground and demand assent to another statement. You know that though the offspring of all plants and animals is in the main like the parent, yet that in almost every instance slight deviations occur, and that sometimes there is even considerable divergence from the parent type. It must also be admitted that these slight variations are often, or at least sometimes, capable of being perpetuated by inheritance. Indeed, it is only in consequence of this fact that our sheep and cattle have been capable of so much improvement. C. I admit this. F. Then the whole matter lies in a nutshell. Suppose that hundreds of millions of years ago there existed upon this earth a single primordial form of the very lowest life, or suppose that three or four such primordial forms existed. Change of climate, of food, of any of the circumstances which surrounded any member of this first and lowest class of life would tend to alter it in some slight manner, and the alteration would have a tendency to perpetuate itself by inheritance. Many failures would doubtless occur, but with the lapse of time slight deviations would undoubtedly become permanent and inheritable, those alone being perpetuated which were beneficial to individuals in whom they appeared. Repeat the process with each deviation and we shall again obtain divergences (in the course of ages) differing more strongly from the ancestral form, and again those that enable their possessor to struggle for existence most efficiently will be preserved. Repeat this process for millions and millions of years, and, as it is impossible to assign any limit to variability, it would seem as though the present diversities of species must certainly have come about sooner or later, and that other divergences will continue to come about to the end of time. The great agent in this development of life has been competition. This has culled species after species, and secured that those alone should survive which were best fitted for the conditions by which they found themselves surrounded. Endeavour to take a bird’s-eye view of the whole matter.