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" In a state of nature almost every plant produces seed, and amongst 

 animals there are very few which do not annually pair. Hence we may 

 confidently assert, that all plants and animals are tending to increase in a 

 geometrical ratio, that all would most rapidly stock every station in which 

 they could any how exist, and that the geometrical tendency to increase must 

 be checked by destruction at some period of life. Our familiarity with the 

 larger domestic animals tends, I think, to mislead lis ; we see no great destruc- 

 tion falling on them, and we forget that thousands are annually slaughtered 

 for food, and that in a state of nature an equal number would have somehow 

 to be disposed of. 



" The only difference between organisms which annually produce eggs or 

 seeds by the thousand, and those which produce extremely few, is, that the slow 

 breeders would require a few more years to people, under favourable conditions, 

 a whole district, let it be ever so large. The condor lays a couple of eggs, and 

 the ostrich a score, and yet in the same country the condor may be the more 

 numerous of the two : the Fulmar petrel lays but one egg, yet it is believed to 

 be the most numerous bird in the world. One fly deposits hundreds of eggs, 

 and another, like the hippobosca, a single one ; but this difference does not 

 determine how many individuals of the two species can be supported in a 

 district. A large number of eggs is of some importance to those species which 

 depends on a rapidly fluctuating amount of food, for it allows them rapidly to 

 increase in number. But the real importance of a large number of eggs or 

 seeds is to make up for much destruction at some period of life ; and this 

 period in the great majority of cases is an early one. If an animal can in any 

 way protect its own eggs or young, a small number may be produced, and yet 

 the average stock be fully kept up ; but if many eggs or young are destroyed, 

 many must be produced, or the species will become extinct. It would suffice 

 to keep up fully the numbers of a tree, which lived on an average for a thousand 

 years, if a single seed were produced once in a thousand years, supposing that 

 this seed were never destroyed, and could be ensured to germinate in a fitting 

 place. So that in all cases, the average number of any animal or plant depends 

 only indirectly on the number of its eggs or seeds." 



The author then proceeds to comment upon the causes which check the 

 natural tendency of each species to increase in number, and points out not only 

 the extreme obscurity of these causes, but also that even when at all ascertain- 

 able, they are found to be very complex and unexpected. Of this he gives 

 several striking instances, as, for example : — 



" 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 effects on the insects must have been still greater, for six 

 insectiverous birds were very common in the plantations, which could not be 

 found on the heath ; and the heath was frequented by two or three distinct 

 insectiverous birds. Here we see how potent has been the effect of the intro- 

 duction 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 Suri'ey. 

 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, 



