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vigorous； or those which have got least food through the advancing winter； which will suffer most。 When we travel from south to north； or from a damp region to a dry； we invariably see some species gradually getting rarer and rarer； and finally 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： we 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 inhabitants； the other species will 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 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 competitors； 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 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。
When a species； owing to highly favourable circumstances； increases inordinately in numbers in a small tract； epidemics at least； this seems generally to occur with our game 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 amongst 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 birds； though having a superabundance 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 plants in a garden； I have in this case lost every single seed。 This view of the necessity of a large stock of the same species for its preservation； explains； I believe； some singular facts in nature； such as that of very rare plants being sometimes extremely abundant in the few spots where they do occur； and that of some social plants being social； that is； abounding in individuals； even on the extreme confines of their range。 For in such cases； we may believe； that a plant could exist only where the conditions of its life were so favourable that many could exist together； and thus save each other from utter destruction。 I should add that the good effects of frequent intercrossing； and the ill 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 unexpected are the checks and relations between organic beings； 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； on the estate 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 been enclosed twenty…five years previously and planted with Scotch fir。 The change in the native vegetation of the planted part of the heath was most remarkable； more than is generally seen in passing from one quite different soil to another： not only the proportional numbers of the heath…plants were wholly changed； but twelve species of plants （not counting grasses and carices） flourished in the plantations； which 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 heath； and the heath was frequented by two or three distinct insectivorous birds。 Here we see how potent has been the effect of the introduction of a single tree； nothing whatever else having been done； with the exception that the land had been enclosed； so that cattle could not enter。 But how important an element enclosure is； I plainly saw near Farnham； in Surrey。 Here there are extensive heaths； with a few clumps of old Scotch firs on the distant hill…tops： within the last ten years large spaces have been enclosed； and self…sown firs are now springing up in multitudes； so close together that all cannot live。 When I ascertained that these young trees had not been sown or planted； I was so much surprised at their numbers that I went to several points of view； whence I could examine hundreds of acres of the unenclosed heath； and literally I could not see a single Scotch fir； except the old planted clumps。 But on looking closely between the stems of the heath； I found a multitude of seedlings and little trees； which had been perpetually browsed down by the cattle。 In one square yard； at a point some hundreds yards distant from one of the old clumps； I counted thirty…two little trees； and one of them； judging from the rings of growth； had during twenty…six years tried to raise its head above the stems of the heath； and had failed。 No wonder that； as soon as the land was enclosed； it became thickly clothed with vigorously growing young firs。 Yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food。
Here we see that cattle absolutely determine the existence of the Scotch fir； but in several parts of the world insects determine the existence of cattle。 Perhaps Paraguay offers the most curious instance of this； for here neither cattle nor horses nor dogs have ever run wild； though they swarm southward and northward in a feral state； and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly； which lays its eggs in the navels of these animals when first born。 The increase of these flies； numerous as they are； must be habitually checked by some means； probably by birds。 Hence； if certain insectivorous birds （whose numbers are probably regulated by h