Although not written with the expectation of publication, I will take the liberty of publishing the following private letter just received from Prof. Leidy. He has not appeared at all in this controversy before the public, and we may safely say that no one can be better qualified than he is to express an opinion on this question of species.
"With all the contention about the question of what constitutes a species, there appears to be almost no difficulty, comparatively, in its practical recognition. Species of plants and animals are daily determined, and the characters which are given to distinguish them are viewed by the great body of naturalists as sufficient. All the definitions, however, which have been given for a species, are objectionable. Morton says: 'A species is a primordial organic form.' But how shall we distinguish the latter? How can it be proved that any existing forms primordially were distinct? In my attempted definition, I think, I fail, for I only direct how species are discovered.
"According to the practical determination of a species by naturalists, in a late number of the Proceedings of our Academy (vol. vii. p. 201), I observe: 'A species is a mere convenient word with which naturalists empirically designate groups of organized beings possessing characters of comparative constancy, as far as historic experience has guided them in giving due weight to such constancy.'
"According to this definition, the races of men are evidently distinct species. But it may be said that the definition is given to suit the circumstances. So it is, and so it should be; or, if not, then all characterized species should conform to an arbitrary definition. The species of gypætus, haliætus, tanagra, and of many other genera of birds, are no more distinguishable than the species of men; and, I repeat, the anatomy of one species of haliætus, or of any other genus, will answer for that of all the other species of the same genus. The same is the case with mammals. One species of felis, ursus, or equus will give the exact anatomy of all the other species in each genus, just as you may study the anatomy of the white man upon the black man. While Prof. Richard Owen will compare the orang with man, and therefore deduce all races of the latter to be of one species, he divides the genus cervus into several other genera, and yet there is no difference in their internal anatomy; while he considers the horse and the ass as two distinct genera, and says that a certain fossil horse-tooth, carefully compared with the corresponding tooth of the recent horse, showed no differences, excepting in being a little more curved, he considers it a distinct species, under the name of equus curvidens; and yet, with differences of greater value in the jaws of the negro and white man, he considers them the same.
"In the restricted genera of vertebrata of modern naturalists, the specific characters are founded on the external appendages, for the most part—differences in the scales, horns, antlers, feathers, hairs, or bills. Just as you separate the black and white man by the difference in the color of the skin and the character of the hair, so do we separate the species of bears, or cats, &c.
"Philadelphia, April 18, 1855."
We might thus go on and multiply, to the extent of an octavo volume, evidence to show how vague and unsettled is the term species among naturalists, and that, when we abandon historical records, we have no reliable guide left. Moreover, were we able to establish perfectly reliable landmarks between species, we still have no means of determining whether they were originally created in one pair, or many pairs. The latter is certainly the most rational supposition: there is every reason to believe that the earth and the sea brought forth "abundantly" of each species.
It must be clear to the reader, from the evidence above adduced, that Dr. Bachman claims far too much when he asserts that—
"Naturalists can be found, in Europe and America, who, without any vain boast, can distinguish every species of bird and quadruped on their separate continents; and the characters which distinguish and separate the several species are as distinct and infallible as are those which form the genera."[200]
"From the opportunities we have enjoyed in the examination of the varieties and species of domesticated quadrupeds and birds, we have never found any difficulty in deciding on the species to which these varieties belong."
Those of us who are still groping in darkness certainly have a right to ask who are the authorities alluded to, and what are those "characters which distinguish and separate species" as distinctly and infallibly as "genera?" They are certainly not in print.
The doctor must pardon us for reminding him that there is printed evidence that his own mind is not always free from doubts. In the introduction of Audubon and Bachman's Quadrupeds of America, p. vii., it is said:—
"Although genera may be easily ascertained by the forms and dental arrangements peculiar to each, many species so nearly approach each other in size, while they are so variable in color, that it is exceedingly difficult to separate them with positive certainty."