From art, then, Darwin harks back once more to nature. He proceeds to show that variability occurs among all wild plants and animals, though not so frequently under ordinary circumstances as in the case of domesticated species. Individual differences everywhere occur between plant and plant, between animal and animal. Sometimes these differences are so very numerous that it is impossible to divide the individuals at all into well-marked kinds; for example, among British wild-roses, brambles, hawkweeds and epilobes, with a few other very variable families, Babington makes as many as 251 distinct species, where Bentham gives only 112--a margin of 139 doubtful forms of shadowy indefiniteness. Varieties, in fact, are always arising, and dominant species in particular always tend to vary most in every direction. The reason why variation is not so marked in the wild state as under domestication is of course because the conditions are there less diverse; but where the conditions of wild things are most diverse, as in the case of dominant kinds, which range over a wide space of country or of ocean, abundant individual variations habitually occur. Local varieties thus produced are regarded by Darwin as incipient species: they are the raw material on which natural selection gradually exerts itself in the struggle for existence.
Granting individual variability, then, how do species arise in nature? And how are all the exquisite adaptations of part to whole, and of whole to environment, gradually initiated, improved, and perfected?
Here Malthus and the struggle for life come in to help us.
For the world is perpetually over-populated. It is not, as many good people fearfully imagine, on a half-comprehension of the Malthusian principle, shortly going to be over-populated; it is now, it has always been, and it will always be, pressed close up to the utmost possible limit of population. Reproduction is everywhere and in all species for ever outrunning means of subsistence; and starvation or competition is for ever keeping down the number of the offspring to the level of the average or normal supply of raw material. A single red campion produces in a year three thousand seeds; but there are not this year three thousand times as many red campions as there were last summer, nor will there be three thousand times as many more in the succeeding season. The roe of a cod contains sometimes nearly ten million eggs; but supposing each of these produced a young fish which arrived at maturity, the whole sea would immediately become a solid mass of closely packed codfish. Linnæus reckoned that if an annual plant had two seeds, each of which produced two seedlings in the succeeding season, and so on continually, in twenty years their progeny would amount to a million plants. A struggle for existence necessarily results from this universal tendency of animals and plants to increase faster than the means of subsistence, whether those means be food, as in the first case, or carbonic acid, water, and sunshine as in the second. Animals are all perpetually battling with one another for the food-supply of the moment; plants are perpetually battling with one another for their share of the soil, the rainfall, and the sunshine.