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 



