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Nov/54/

Theoretical Geograph. Distrib. (18)

A species is well suited to its conditions, sports & became modified or becomes parent of species either remaining itself generally for a time, & then usually replaced & dying: but facts of one new species having been formed is evidence that it is suited to conditions, & will probably give rise to other forms. If the region will support so many Compositæ, which one genus, has sported & shown its adaptation is the most likely to yield more forms.

Or this, When a species breaks & gives rise to another species, the chances seem favourable (for it has given birth to one simply because its whole constitution is well adapted to this condition &) to its giving birth to others (No doubt here comes in question of how far isolation is necessary, & I shd have thought more necessary then facts seems to show it) In fact there never can be isolation for the parent form must always be present & tend to cross & bring back, the ancestral form; it will always be a struggle against crossing, & require either vigorous selection or some isolation from habits, nature of country to separate) Hence, small, genera will be local owing to their origins from common parent; & small genera (ie with 2-8 species,) certainly, from Shöenherr, are local in proportions of 215:52 (& these 52 are not such small genera as the others).

As to make species is slow work, 1 genus increase to considerable size much time wd be required, hence as Forbes says wd be local in their origins in past times; the species wd extend over continuous spaces in area & time. But it is generally happens during the time necessary to make a large genus, that geographical mutations & chance accident wd disperse genera & the very fact of the genus having become large in an area, we may suppose wd give it some better chance in another continuous area, & thus the genus wd get bigger & bigger. And certainly most large genera are widely extended. When a genus began to fail & die out, if large, it wd leave probably a few species in distant quarters of the world: Hence this wd be another cause of small genera; these wd. be aberrant

This mere fact of large genera, generally having wider

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BBB 1856 Feb

It is clear these are two very distinct causes for small genera, just forming & becoming extinct; the latter in affinity will have species very different from each other, & will be more apt to have wide ranges - Babington I remember remarks that when a genus has its [illeg] the species are apt to run into each other; where, I think, discussing in Annals of Nat Hist 1855 the Batrachian Ranunculi. But yet; as I have shown, the small genera with widest ranges are not the most aberrant, but yet they may be broken genera with their few species not very closely allied.

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can be accounted for by creationist, showing that if a genus be created in different district question it wd probably form so many local species.

If inhabitants of S. America & Australia turned into each other by [illeg] I shd expect the increasing larger genera of the two wd persist & spread & exterminate many of the smaller genera; probably inhabitants of one continent wd prevail considerably over the other; when one continent formed another, usually one wd be sinking & decreasing.

Unusual powers of dispersion (mem. all insects are bisexual) might account for some smaller genera, being widely distributed, but I am much surprised, if small genera with widely distributed species are not remnants of large genera & so as species aberrant or very distant from each other, though they may not belong to aberrant genera. I think thus alone we can account for "wandering species" of plants being generally very distinct, they are remnants after extinction of connecting species.

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All existing continents show signs of former connections with other continents.

Strictly it is the large genera which have arrived at these numbers from conditions having been favourable they may be now increasing or decreasing. The increasing genera are the very returning genera with close species.

Ought there not to be as many small genera due to forming genera, as to dying genera. Yes I think (?)

The dying genera will often be widely spread & will contain more distinct species.