From Edward Blyth 8 October 1855
Octr 8th 1855—
My dear Sir,
By the mail which leaves this day I have the pleasure to send you a copy of my printed report (just published) on a zooll collection from the Somáli country,1 and also a lot of M.S. for your consideration.2 These you will receive through Dr Horsfield. The said MS. consists, firstly, of doz. sheets like this one, devoted to the more immediate subject of your present enquiries, which is treated more elaborately & systematically than before, in consideration of which you must pardon some repetition; & secondly, of doz. more such sheets (putting your patience a little to the test, surely,) devoted to a commentary upon the 2d Vol. of Lyell’s ‘Principles’,3 with collateral matters that occurred to me. I fancy that I have not yet quite done with Lyell, & may find time to look over his other Vols., & also certain other works, ere the next mail leaves us. With regard to the vexata quæstio of species, & notwithstanding the occasional occurrence of prolific hybrids, we still cannot but admit that, as a very general rule, species do not intermix in wild nature, as varieties of the same species (descendants from a common stock) we have every reason to think would do. Among gregarious animals however, as especially cattle, the members of the same herd well know each other, & will not readily permit an intruder from another herd to associate with them; this would help to keep varieties distinct; & then we have the fact that the most powerful male keeps other males at a distance, & so monopolizes the propagation; whereby it follows, that whilst the best blood is thus ordinarily transmitted, at the same time any peculiarity would likewise be extensively transmitted; & hence perhaps the origin of certain races, or what I have termed normal as distinguished from abnormal varieties. The influence of the same leading male might well extend over the generation of a succession of seasons, & thus permanently affect the future herd. Meanwhile, the other large males driven from the herd would mostly combat and destroy each other, or fall a prey to their natural enemies; comparatively few becoming the centres of a new circle of feminine acquaintances. On the other hand, we must remember that, in general, the male distinguished by some marked peculiarity is not the most likely one to become the patriarch of the herd! Next, it may be remarked that of species so nearly affined as to be liable to interbreed, (e.g. the humped & humpless cattle), either there are geographical difficulties to be first overcome, or, should such not present themselves, originally distinct races may have fused more or less completely long ago, & so have given origin to what have therefore come to be considered as single species, & especially to some of the more variable of our present supposed species—the Horse for instance. I could never perceive that it necessarily follows that species or races of independent origin (so far at least as one can infer this in any case) should differ at all!
In races from the most distant regions, and where we have certainly no reason to suppose that the particular species had passed from one to the other, we have every grade of approximation from the most obvious & universally acknowledged to be distinct, to undistinguishable (if not absolute) similarity. Take the Skua Gull of Australia as compared with that of the north; & this bird has never been seen within the tropics. The Great Grebes of Europe & Australia are a more doubtful case, for I have obtained the Podiceps cristatus from the Bengal Soonderbans. Or turn to Gould’s ‘Birds of Australia’. There you will find the Falcunculus of the east represented by a corresponding but obviously distinct species in the west; & many more cases of the kind. Then each principal region has its own peculiar black Cockatoo, which Gould figures as distinct;4 but the white C. galerita is common to all the regions & to N. Guinea, &c, but presents differences of size &c in different localities, indicating perhaps a plurality of races actually as distinct apart as the black one, but manifesting a still closer approximation. This is even the more likely, as we recognise the same peculiar type in the small sulphur-crested C. sulphurea of Timor, & the new C. citrino-cristata (which I have) the precise native region of which is still unknown. We can only weigh probabilities. We cannot but recognise a certain law, by which it appears that different regions (and especially those subordinate to the same zoological or botanical province) present series of species which, though admitted on all hands to be distinct & peculiar, nevertheless represent other species in the corresponding regions; the different provinces of Australia for instance; & the tropical regions E & W. of the Andes. Here we have even so remarkable a form as Rupicola represented on both sides of the chain; and numerous corresponding species of Cotinga, Tanager, &c (confining our attention to the bird class for brevity). Among such corresponding species there is every grade of approximation until all apparent distinctness is lost; & yet analogy would often indicate that such undistinguishable forms are in reality as distinct as the rest; & it is likely (I think) that not only such, but some of those corresponding races which are more or less distinguishable, would intermingle & fade were the geographical difficulties to be surmounted.— Still the phenomenon of prolific hybrids is exceptional; as remarkably shewn by my hybrids from Gallus Sonneratii; all the numerous eggs produced from or by the agency of which, with male or female of the domestic fowl, proved invariably abortive.5 Varieties of the same species we presume to be always prolific inter se: but even on this subject, consult Hutton in the ‘Calcutta Journal of Nat. History’, description of Capra ægagrus,6 which I still think is the manifest origin of the domestic Goat. It would even seem that the latter interbreeds with the Swiss Ibex, a far more different species than the ægagrus; & it is said that F. Cuvier’s figures of (or assigned to) the ægagrus, m. & f., with their young, represent these hybrid Ibices!7 But sterile individuals of pure species (both male & female) occur not so very unfrequently, and merely appear to be more common among hybrids, & among some than other hybrids. How about the Red Deer, which is said to be dying off from sterility both in Ireland and upon Dartmoor? Enquire into this. And do you remember the extraordinary case of occasional migration which occurred among the Red Deer, I think of Sutherlandshire, & of which a very interesting account was given in Chambers’s Journal? 8 Reverting to the case of affined species, no end of instances occur to me, in sundry classes, wherein it is not likely that naturalist will ever approximately come to agreement. There is the common Tree-frog of Bengal Polypedotes leucomystax., which here does not vary much, yet seems to differ in almost every district from which I have received it—various parts of S. India, Ceylon, Sylhet, Burma, Malasia, &c; in every locality there seems to be a peculiarity in the colouring. I doubt not that the Prince of Canino would make many species of it; Schlegel but one:9 & then again, surrounding this nucleus (of local varieties?), we have many other races closely affined, but more or less satisfactorily distinguishable.
Then among shells, many of the large Cyclostomata & Melaniæ of India & neighbouring countries; but, Mr W. Clark (as I see in the ‘Zoologist’ for July last, p. 4758)10 is carrying the war into the camp of the mere conchologists! I have only quite recently seen this publication for the first time, and as yet only the Nos. for the current year: but among these I must call your attention to Dr Knox’s remarks on species (p. 4791).11 Knox is an original thinker, and amusingly indisposed to hide his light under a bushel; moreover, not a little opinionated in some matters, & few have better grounds on which to form opinions. But I demur to his dogma about gaps (l. 10 from bottom).12 Pretty much as a tree branches off, & still divides & subdivides & resubdivides, and with almost as much irregularity, so I conceive that the successively subordinate groups of organisms divide off from each other without anastomosing or coalescing; & that accordingly there are no links connecting the twigs of the different branches, which in gardener’s phrase are never “inarched”13 , but remain independent of each other save insofar as they alike ramify from one primal stem or type. Thus the principal branches represent classes, & so on.
With kindest regards, I remain | Yours ever truly, E Blyth
P.S. I meant to have said a few words about the geographic range of various wading & swimming birds; suggested by the erroneous statement in Lyell (quoted from Bewick) about that of the wild Anser cinereus.14 A considerable proportion of the waders & swimmers of the Old World have the same range as Anser cinereus; others as the Mallard & Pintail Ducks extend also to America; & the latter is very common in India, while the Mallard here keeps to the vicinity of the Himalaya. Are any save tropical Anatidæ common to the N. & S. hemispheres? I think not. The Greenshank is common to Britain & Australia, I do not admit the distinctness of glottoides. with all the intervening lands. Do. Black-tailed Godwit.
Encloses two sets of notes [see 1761 and 1762]. EB believes that as a general rule species do not inter-mix in nature whereas varieties, descendants of a common stock, do. Origin of varieties. Geographically separated species are sometimes obviously distinct and sometimes apparently identical. EB does not believe that species or races of independent origin need necessarily differ. Local distribution of species of black cockatoo contrasts with the widespread white cockatoo. The occurrence of distinct but related species in different regions of a zoological province, preserved because of geographical barriers. Instances of interspecific hybrids and intraspecific sterility. Local varieties of species. Varieties are subdivisions of the main branches of the tree of organisms, dividing irregularly but remaining independent of the twigs from another branch.
Please cite as
Darwin Correspondence Project, “Letter no. 1760,” accessed on 22 October 2016, http://www.darwinproject.ac.uk/DCP-LETT-1760