All the while Darwin was working on these books his mind was quietly busying itself with what he called the species question. The more he studied the material collected on his long tour, the more confident he became that the animals of the present are the altered descendants of the animals of the past. He tried patiently to work out every conceivable hypothesis to see whether he could account for the alteration. He felt quite sure animals changed, but how they changed, and why, he could not for a long time conceive. He knew that gardeners were constantly producing new varieties of plants, and that animals of various breeds were clearly the descendants of other and familiar varieties. Accordingly he began to study the methods of animal and plant breeders, to visit their farms, to open correspondence with them and read all their trade journals, to undertake experiments in the breeding of plants. The longer he worked the more confident he became of the reality of the change; but for a long time no glimmer of the cause by which it could be brought about came to his mind. In 1838 he came across a book by Malthus called "An Essay on Population," in which the author shows that, whereas man increases by a geometric ratio, he cannot hope to increase his food supply in more than an arithmetic ratio. That is, while the food might increase like the series 2-4-6-8-10, the population would increase like the series 2-4-8-16-32. On this basis it is only a question of time when the earth will be too full of people for it to be possible for the food to sustain them. Malthus added many observations and suggestions, but this is as much of the book as interests us in this connection. Here was the idea that suggested to Darwin his agency for producing the change of the animals of the past into those of the present. The number of animals of any particular species remains practically the same. There may be a few more one year, and a few less another, but on the average, year by year, the number of toads, the number of blacksnakes, the number of field mice, remains sensibly the same. Sometimes the rise of man brings an end to the wild population, and so in the past animals have dropped out of the race. Yet in the long run and for a considerable time the number of any species is constant. But each animal produces offspring in quantities sufficient to far more than replace himself as he dies out. In other words, animals increase not by addition but by multiplication. Too many are born for all of them to live. What becomes of the great mass of them? The answer is they die; most of them die young. Only a few fortunate individuals, favored by being a little stronger, a little more cunning, a little more attractively colored than their mates, survive to carry on the race.