The question is, whether in nature there are causes competent to produce 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 OF 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, which we have seen they require; then these are all, at least the two first, what we may call the inorganic or physical conditions of existence.Food takes a mid-place, and then come the organic conditions; by which I mean the conditions which depend upon the state of the rest of the organic creation, upon the number and kind of living beings, with which an animal is surrounded.You may class these under two heads: there are organic beings, which operate as 'opponents', and there are organic beings which operate as 'helpers' to any given organic creature.The opponents may be of two kinds: there are the 'indirect opponents', which are what we may call 'rivals'; and there are the 'direct opponents', those which strive to destroy the creature; and these we call 'enemies'.By rivals I mean, of course, in the case of plants, those which require for their support the same kind of soil and station, and, among animals, those which require the same kind of station, or food, or climate; those are the indirect opponents;the direct opponents are, of course, those which prey upon an animal or vegetable.The 'helpers' may also be regarded as direct and indirect:
in the case of a carnivorous animal, for example, a particular herbaceous plant may in multiplying be an indirect helper, by enabling the herbivora on which the carnivore preys to get more food, and thus to nourish the carnivore more abundantly; the direct helper may be best illustrated by reference to some parasitic creature, such as the tape-worm.The tape-worm exists in the human intestines, so that the fewer there are of men the fewer there will be of tape-worms, other things being alike.It is a humiliating reflection, perhaps, that we may be classed as direct helpers to the tape-worm, but the fact is so:
we can all see that if there were no men there would be no tape-worms.
It is extremely difficult to estimate, in a proper way, the importance and the working of the Conditions of Existence.I do not think there were any of us who had the remotest notion of properly estimating them until the publication of Mr.Darwin's work, which has placed them before us with remarkable clearness; and I must endeavour, as far as Ican in my own fashion, to give you some notion of how they work.We shall find it easiest to take a simple case, and one as free as possible from every kind of complication.
I will suppose, therefore, that all the habitable part of this globe--the dry land, amounting to about 51,000,000 square miles,--Iwill suppose that the whole of that dry land has the same climate, and that it is composed of the same kind of rock or soil, so that there will be the same station everywhere; we thus get rid of the peculiar influence of different climates and stations.I will then imagine that there shall be but one organic being in the world, and that shall be a plant.In this we start fair.Its food is to be carbonic acid, water and ammonia, and the saline matters in the soil, which are, by the supposition, everywhere alike.We take one single plant, with no opponents, no helpers, and no rivals; it is to be a "fair field, and no favour".Now, I will ask you to imagine further that it shall be a plant which shall produce every year fifty seeds, which is a very moderate number for a plant to produce; and that, by the action of the winds and currents, these seeds shall be equally and gradually distributed over the whole surface of the land.I want you now to trace out what will occur, and you will observe that I am not talking fallaciously any more than a mathematician does when he expounds his problem.If you show that the conditions of your problem are such as may actually occur in nature and do not transgress any of the known laws of nature in working out your proposition, then you are as safe in the conclusion you arrive at as is the mathematician in arriving at the solution of his problem.In science, the only way of getting rid of the complications with which a subject of this kind is environed, is to work in this deductive method.What will be the result, then? I will suppose that every plant requires one square foot of ground to live upon; and the result will be that, in the course of nine years, the plant will have occupied every single available spot in the whole globe! I have chalked upon the blackboard the figures by which Iarrive at the result:-
