I remember putting the case to myself almost in your words,—Why does not every individual become converted into distinct species? The answer, which I framed, and the discussion on this curious point, which I have somewhere in M.S. is nearly to the following effect: Although the number of species supported over equal areas under apparently nearly similar conditions is now very different in different quarters of the world; and although we are far from knowing that the most prolific area is fully stocked with species, as perhaps may be inferred if not so from some European plants having become naturalized even at Cape of Good Hope, yet Geology shows us, at least within the whole immense Tertiary period, that the number of species of shells & probably of mammals has not increased. How is this? As far as mere physical conditions are concerned (i.e. heat, moisture, height, etc., etc.) it seems probable that a sufficient number of species would soon become sufficiently adapted to the most diversified conditions, but the relation of organic being to organic being, I fully admit is far more important, and as the principle of divergence goes on augmenting the number of species in any area, or in the world, the relation between being and being will become more and more complex, and there seems at first no limit to the amount of possible diversification of structure and therefore no limit to the number of species. But I think the following causes will tend to limit the number (1) the amount of life (not number of specific forms) supportable on any area cannot be illimitable, depending so largely as it does on physical conditions; and therefore where very many species are supported, each or nearly each will be few in individuals; and any species with scanty numbers will be liable to extermination from accidental fluctuations in seasons and number of enemies. The process of exter mination would in such case be rapid, whereas the process of production of new species would always be slow. Just imagine the extreme case of as many species as individuals in England, and the first severe winter or very dry summer would exterminate forever thousands on thousands of species. (2) I suspect that when any species becomes very rare, close interbreeding will tend to exterminate it; at least authors have thought that this comes into play in accounting for the deterioration of Aurochs in Lithuania and of Red Deer in Scotland, Bears in Norway, etc.³ (3) As far as animals are concerned some species owe their origin to being adapted to prey on some one other being, but if this other one being be rare, it will not have any advantage to an animal to have been produced in close relation to it, whilst so poor in number, and therefore it will not have been produced. (4) When any species becomes few in number, the process of modification will be slower for the chance of favourable variations arising will be lessened. Therefore if we suppose all or most of the species in any area to be very numerous and consequently each poor in individuals, the proofs of modification and of giving birth to new forms will probably be retarded. (5) And this I am inclined to think is the most important element: a dominant species which has already beaten many competitors will tend to beat more; the relation of organism to organism being more important than relation to physical conditions; and Alph. de Candolle has shown that those species which spread widely tend generally to spread very widely,⁴ and consequently they will tend to supplant several species in several areas and thus check the inordinate increase of specific forms throughout the world. Hooker has shown that in S.E. corner of Australia, where there are many invaders, (i.e. dominant forms) from different quarters of the world, the endemic Australian species have apparently been greatly reduced.⁵

How much weight to attribute to these several causes I do not pretend to assign; but conjointly I think they must limit the constant tendency to augmentation of specific forms. What do you think? I should like to hear.⁶

With respect to “convergence”⁷ I daresay, it has occurred, but I should think on a very limited scale, (owing to strong principle of inheritance retaining resemblance to parents) and only in case of closely related forms.⁸ In case of varieties and subvarieties of cultivated plants, I dare say (I have vague remembrance that I have heard such a case) that the offspring of one variety might easily so closely resemble the offspring of a distinct variety as to be classed as a subvariety of it, instead of its own parent. When thinking of analogical or adaptive characters, as of external resemblance of whale and fish, it occurred to me that the same cause acting on two closely related forms (i.e. those which closely resembled each other from inheritance from a common parent) might confound them together so closely that they would be (falsely in my opinion) classed in same group. But I should require pretty strong evidence before I should be willing to admit that this had often occurred.—⁹

C. Darwin