[1]

The sketch supports the belief that species are the descendants of a common ancestor and this implies divergence — therefore he certainly recognized the fact of divergence tho' he says he did not perhaps see why divergence was beneficial—

Or put it in another way he compared what goes on in nature with the process of breeding the gradual change which an organism undergoes owing to the variations in a given direction being constantly selected— But if 2 breeders men breed in opposite directions — as in the case of the carthorse & race horse

2

you get divergence — Tho' you do not get such strong divergence as under nature — for in such a case as the horse the intermed forms of horse are useful for intermed purposes — whereas in nature it would happen that only the extremes would survive — This survival of the extremes & extinction of the intermediate forms he recognized in 1844 — witness the discussion on cabbage — Supposing there to be two distinct of distinct families genera A and Z inhabiting a region and that A has one species Ab, while of Z there are two closely allied sp Zx and Zy. Now let the conditions change — eg by a great increase of lakes and rivers so that

[in margin:] say a rodent and a marsupial

3

there is more room for an aquatic anim & less for land animals — We may suppose (i) that Ab becomes modified into an aquatic form subgenus B that the varieties of Ab which do not become aquatic are extinct. In the mean while neither of the Z species has been able to vary in the aquatic direction, and in consequence Zx has become extinct, while Zy still lives as a land animal — say Zy'

ii We may imagine that Aa has not been able to vary in the aquatic direction and becomes extinct, but that Zx has varies and so much that it deserves to be call a new genus X, while the descendents of Zy live one perhaps slightly modified as a land animal Zy'—

Here then there is divergence — in i there was not — since the survivors A B and Zy

[in margin:] being beaten out by Zy

4

are no further apart in the classification sense than they were —

In case ii we have the genera Z and X both – descended from the old genus Z.

It is clear however that whether there is divergence or not — the process is the same — viz the survival of the two forms most able to adapt themselves to water and land stations —

It seems to me that divergence is a consequence of success and not vice versa

Any given station manner of life can only support a certain number of individuals — therefore if a species the descendants of a certain form now adapted to this manner of life are ever to exceed 1/1000 of the fauna population — they must do so

[in margin:] say 1/1000 of the fauna whole number of organism living in that region —

5

by becoming seizing on other stations — either unocc or occupied from which they oust the former occupants — The only criterion of success in a species is increased in the number of it descendants which live — \ success in this sense depend on be accompanied by divergence

This is given in origin p 126

[Origin, p. 126: "Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which as yet have suffered least extinction, will for a long period continue to increase. But which groups will ultimately prevail, no man can predict; for we well know that many groups, formerly most extensively developed, have now become extinct. Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that of the species living at any one period, extremely few will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on Classification, but I may add that on this view of extremely few of the more ancient species having transmitted descendants, and on the view of all the descendants of the same species making a class, we can understand how it is that there exist but very few classes in each main division of the animal and vegetable kingdoms. Although extremely few of the most ancient species may now have living and modified descendants, yet at the most remote geological period, the earth may have been as well peopled with many species of many genera, families, orders, and classes, as at the present day."]

Or thus:— each species produces enormously by greater number of offspring than can occupy the station held by that species —

eg Thistles in s. America — The offspring are

[6]

Rats 5e

R 192 200

39 in 15' [x] 4 [=] 15.6 per hr