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Show 68 CHECKS TO INCREASE. [CllAP, III. we travel south·ward and see a species decreasing in numbers, we may feel sure 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 therefore of con1petitors, decreases northwards; hence in going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding south' vards 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 elmnents. That clhnate acts in Inain part indirectly by favouring other species, we n1ay 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 coinpete with our native plants, nor resist destruction by our native anin1als. When a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract, epidmnics-at least, this seems generally to occur with our gaine animals-often ensue : and here we have a limiting check independent 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 facility of diffusion ainongst the crowded animals, been disproportionably 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 individuals 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 bird~ though having a super-abundance of food at this one season, 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 from a few wheat or other such llHAP. III.] MUTUAL CHECKS TO INCREASE. 69 plants in a garden ; I have in this case lost every single seed. This view of the necessity of a large stock of the s~me species for its preservation, explains, I believe, some sn~gular fact~ in nature, ~uch as that of very rare plants bmng sometimes extremely abundant in the few spots wh~re they do occur; and that of some social plants being social, tha~ is, abounding in individuals, even on the extre: ne confines of their range.. For in such cases, we may bel;ev~, that a plant could exist only where the conditions of Its hfe "''"ere so favourable that many could exist together, and thus save each other from utter destruction. I should ~dd that the good e~ects of frequent intercrossing, ~nd the Il~ effects of close Interbreeding, probably come Into. play In some of these cases ; but on this intricate subJect I will not here enlarge. Many cases are on record showing how complex and un~xpected. are the checks and relations between organic bmngs, which have to struggle together in the same country. I will give only a single instance which though a simple one, has interested me. In Staffordshire: ?n th~ es~ate of a relation where I had ample means of Investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had be.en enclosed twenty-five years previously and planted With Scotch fir. The change in the native vegetation of the planted part of the heath w·as most remarkable, more than is generally seen in passing from o.ne quite different soil to another : not only the propor tional nuinbers of the h~ath plants were wholly changed, but. twelve ~peme~ of plants (not counting grasses and carwes) flourished In the plantations, 'vhich could not be found on the heath. The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the ?e!lth ; . and ~he heath was frequented by two or three distmct Insectivorous birds. Here we see l1ow potent has been the effect of the introduction of a sinO'le tree, nothing whatever else having been done with the exception that the land had been enclosed, so that cattle |