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Show '68 . CHECKS TO INCREASE. CHAP. III . the more vigorous plants gradually kill the less vigorous, though fully grown, plants: thus out of twenty species growing on a little plot of turf (three feet by four) nine species perished from the other species being allowed to grow up freely. The amount of food for each species of course gives the extreme limit to which each can increase ; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin. If not one head of game were shot during the next twenty years in England, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually killed. On the other hand, in some cases, as with the elephant and rhinoceros, none are destroyed by beasts of prey : even the tiger in India most rarely dares to attack a young elephant protected by its dam. Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought, I believe to be the most effective of all checks. I estimated that the winter of 1854-55 destroyed four-fifths of the birds in n1y own grounds ; and this is a tremendous destruction, when we remember that ten per cent. is an extraordinarily severe mortality from epidemics with man. The action of cli: rr:tate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. Even when climate, for instance extreme CHAP. III. CHECKS TO INCREASE. 69' col~, acts directly, it will be the least vigorous, or those wluch have got least food through the advancing winter which will suffer most. When we travel from south t~ north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finall~ disappearing; and the change of climate being conspicuous, we are tempted to attribute the whole effect to its direct action. But this is a very false view : w. e forget that each species, even where it most abounds ' Is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers and as each area is already fully stocked with inhabi't ants,' the other species will decrease. When we travel southward and see a species decreasing in numbers, we may f~el su:e that the cause lies quite as much in other speCies being favoured, 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 therefo~ e of competitors, decreases northwards; hence in gmng north ward, or in ascending a mountain, we far ?f~en.er mee~ with stunted forms, due to the directly InJUriOus actiOn of climate, than we do in proceeding southwards or in descending a mountain. When we 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 may clearly see in the prodigious number of plants in our gardens which can perfectly well endure our climate, but which never become naturalised, for they cannot compete with our native plants, nor resist destruction by our native animals. |
