Page - 84 - in The Origin of Species

84 ORIGIN OF SPECIES as each area is already fully stocked with inhabitants, the other species must decrease. When we travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being fa- voured, as in this one being hurt. So it is when we travel northward, but in a somewhat lesser degree, for the number of species of all kinds, and therefore of competitors, de- creases northwards; hence in going northwards, or in as- cending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southwards or in descending a mountain. When v/e reach the Arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements. That climate acts in main part indirectly by favouring other species, we clearly see in the prodigious number of plants which in our gardens can perfectly well endure our climate, but which never became naturalised, for they can- not compete with our native plants nor resist destruction by our native animals. When a species, owing to highly favoured circumstances, increases inordinately in numbers in a small tract, epidemics —at least, this seems generally to occur with our game ani- mals—often ensue; and here we have a limiting check inde- pendent of the struggle for life. But even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through fa- cility of diffusion amongst the crowded animals, been dis- proportionally favoured : and here comes in a sort of struggle between the parasite and its prey. On the other hand, in many cases, a large stock of indi- viduals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation. Thus we can easily raise plenty of corn and rape-seed, &c., in our fields, because the seeds are in great excess, compared with the number of birds which feed on them ; nor can the birds, though having a superabundance of food at this one sea- son, increase in number proportionally to the supply of seed, as their numbers are checked during winter ; but any one who has tried, knows how troublesome it is to get seed