It is not generally realized how severe is the pressure and competition in Nature--not between different species, but between the immature population of one and the same species, precisely because they are of the same species and have exactly the same needs. From a human point of view the pressure under which many wild things live is awful in its severity and relentless tenacity. Not only are new forms established by natural selection, but the old forms, when they exactly fit the mould presented as it were for competitive filling, are maintained by the same unremitting process. A distinctive quality in the beauty of natural productions (in which man delights) is due to the unobtrusive yet tremendous slaughter of the unfit which is incessantly going on, and the absolute restriction of the privilege of parentage to the happy few who attain to the standard described as ‘the fittest.’ 7. THE LIMITED VARIETY OF NATURE’S PRODUCTS. The process of development of an immense variety of animal and vegetable forms has proceeded in this way through countless ages of geologic time, but it must not be supposed that any and every conceivable form and variety has been produced. There are only two great diverging lines of descent from original living matter--only the animals and the plants. And in each of these there are and have been only a limited number of branches to the pedigree--some coming off at a lower level, others at higher points when more elaborate structure has been attained. It is easy to imagine groups of both plants and animals with characters and structures which have never existed and never will exist. The limitation of the whole process in spite of its enormous duration in time, its gigantic output and variety, is a striking and important fact. Linnaeus said, ‘There are just as many species as in the beginning the Infinite Being created’; and the modern naturalist can go no further than the paraphrase of this, and must say, ‘There are and have been just so many and just so few varieties of animal and vegetable structure on this earth as it was possible for the physical and chemical contents of the still molten globe to form up to the hour now reached.’ 8. THE EMERGENCE OF MAN.