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Show 120 CONDITIONS AFFECTING THE while the tigers of Java and Sumatra differ in many important respects from the tigers of Northern Asia. So lions vary; so birds vary; and so, if you go further back and lower down in creation, you find fishes vary. In different streams, in the same country even, you will find the trout to be quite different to each other and easily recognizable by those who fish in the particular streams. There is the same differences in leeches ; leech collectors can easily point out to you the differences and the peculiarities which you yourself would pro. bably pass by; so with fresh-water mussels; so, in fact, with every animal you can mention. In plants there is the same kind of variation. rrake such a case even as the common bramble. The botanists are all at war about it; some of them wanting to make out that there are many species of it, and others maintaining that they are but many varieties of one species; and they cannot settle to this day which is a species and which is a variety ! So that there can be no doubt whatsoever that any plant and any animal may vary in nature; that varieties may arise in the way I have described,-as spontaneous varieties,-and that those varieties may be perpetuated in the same way that I have shown you spontaneous varieties are perpetuated; I say, therefore, that there can be no doubt:as to the origin and perpetuation of varieties in nature. But the question now is:- Does selection take place in nature? is there anything like the o1Jeration of man in exercising selective breeding, taking place in nature? You will observe that, at present, I say nothing about species; I wish to confine myself to the PERPETUATIO~ OF LIVING BEINGS. 121 consideration of the production of those natural races which everybody admits to exist. The question is, whether in nature there are causes competent to pro .. duce races, just in the same way as man is able to produce, by selection, such races of animals as we have already noticed. When a variety has arisen, the CoNDITIONS 011' ExisTENCE are such as to exercise an influence which is exactly comparable to that of artificial selection. By Conditions of Existence I mean two things,- there are conditions which are furnished by the physical, the inorganic world, and there are conditions of existence which are furnished by the organic world. There is, in the first place, CLIMATE; under that head I include only temperature and the varied amount of moisture of particular places. In the next place there is what is technically called STATION, which means- given the ·climate, the particular kind of place in which an animal or a plant lives or grows; for example, the station of a fish is in the water, of a fresh-water fish in fresh water; the station of a marine fish is in the sea, and a marine animal may have a station higher or deeper. So again with land animals: the differences in their stations are those of different soils and neighbourhoods; some being best adapted to a calcareous, and others to an arenaceous soil. The third condition of existence is FooD, by which I mean food in the broadest sense, the supply of the materials necessary to the existence of an organic being; in the case of a plant the inorganic matters, such as carbonic acid, water, ammonia, and the earthy salts or salines; in the case of the animal the inorganic and organic matters, |