In birds also similar relations exist, but there is less often an increase in size northward. In species whose breeding station covers a wide range of latitude, the northern birds are not only smaller, but have quite different colors, as is markedly the case in the common quail, the meadow-lark, the purple grackle, the red-winged blackbird, the flicker, the towhee bunting, the Carolina dove, and in numerous other species. The same difference is also quite apparent in the blue jay, the crow, in most of the woodpeckers, in the titmice, numerous sparrows, and several warblers and thrushes. The variation often amounts to from ten to fifteen per cent of the average size of the species.
Allen also states that certain parts of the animal may vary proportionately more than the general size, there being an apparent tendency for peripheral parts to enlarge toward the warmer regions, i.e. toward the south. “In mammals which have the external ears largely developed—as in the wolves, foxes, some of the deer, and especially the hares—the larger size of this organ in southern as compared with northern individuals of the same species, is often strikingly apparent.” It is even more apparent in species inhabiting open plains. The ears of the gray rabbit of the plains of western Arizona are twice the size of those of the Eastern states.
In birds the bill especially, but also the claws and tail, is larger in the south. In passing from New England southward to Florida the bill in slender-billed forms becomes larger, longer, more attenuated, and more decurved; while in short-billed forms the southern individuals have thicker and larger bills, although the birds themselves are smaller.
The remarkable changes and gradations of color in birds in different parts of North America are very instructive, and the important results obtained by American ornithologists form an interesting chapter in zoology. The evidence would convince the most sceptical of the difficulty of distinguishing between Linnæan species. It is not surprising to find in this connection a leading ornithologist exclaiming, “if there really are such things as species.” The differences here noted are mainly from east to west. We may briefly review here a few striking cases selected from Coues’s “Key to North American Birds.”
The flicker, or golden-winged woodpecker (Colaptes auratus), has a wide distribution in eastern North America. It is replaced in western North America (from the Rocky Mountains to the Pacific) by C. mexicanus. In the intermediate regions, Missouri and the Rocky Mountain region, the characters of the two are blended in every conceivable degree in different specimens. “Perhaps it is a hybrid, and perhaps it is a transitional form, and doubtless there are no such things as species in Nature.... In the west you will find specimens auratus on one side of the body, mexicanus on the other.” There is a third form, C. chrysoides, with the wings and tail as in auratus, and the head as in mexicanus, that lives in the valley of the Colorado River, Lower California, and southward.
In regard to the song-sparrow (Melospiza), Coues writes: “The type of the genus is the familiar and beloved song-sparrow, a bird of constant characters in the east, but in the west is split into numerous geographical races, some of them looking so different from typical fasciata that they have been considered as distinct species, and even placed in other genera. This differentiation affects not only their color, but the size, relative proportions of parts, and particularly the shape of the bill; and it is sometimes so great, as in the case of M. cinerea, that less dissimilar looking birds are commonly assigned to different genera. Nevertheless the gradation is complete, and affected by imperceptible degrees.... The several degrees of likeness and unlikeness may be thrown into true relief better by some such expressions as the following, than by formal antithetical phrases: (1) The common eastern bird commonly modified in the interior into the duller colored (2) fallax. This in the Pacific watershed, more decidedly modified by deeper coloration,—broader black streaks in (3) hermanni, with its diminutive local race (4) samuelis, and more ruddy shades in (5) guttata northward, increasing in intensity with increased size in (6) rafina. Then the remarkable (7) cinerea, insulated much further apart than any of the others. A former American school would probably have made four ‘good species,’ (1) fasciata, (2) samuelis, (3) rafina, (4) cinerea.”
Somewhat similar relations are found in three other genera of finches. Thus Passerella is “imperfectly differentiated”; Junco is represented by one eastern species, but in the west the stock splits up into numerous forms, “all of which intergrade with each other and with the eastern bird. Almost all late writers have taken a hand at Junco, shuffling them about in the vain attempt to decide which are ‘species’ and which ‘varieties.’ All are either or both, as we may elect to consider them.” In the distribution of the genus Pipilo similar relations are found. There is an eastern form much more distinct from the western forms than these are from each other.
Finally may be mentioned the curious variations in screech-owls of the genus Scops. This owl has two strikingly different plumages—a mottled gray and a reddish brown, which, although very distinct when fully developed, yet “are entirely independent of age, season, or sex.” There is an eastern form, Scops asio, that extends west to the Rocky Mountains. There is a northwestern form, S. kennicotti, which in its red phase is quite different from S. asio, but in its gray plumage is very similar. The California form, S. benderii, is not known to have a red phase, and the gray phase is quite different from that of S. asio, but like the last form. The Colorado form, S. maxwellæ, has no red phase, “but on the contrary the whole plumage is very pale, almost as if bleached, the difference evident in the nestlings even.” The Texas form, S. maselli, has both phases, and is very similar to S. asio. The Florida form is smaller and colored like S. asio. The red phase is the frequent, if not the usual, one. The flammulated form, S. fiammula, is “a very small species, with much the general aspect of an ungrown S. asio.” This is the southwestern form, easily distinguished on account of its small size and color from the other forms.
These examples might be greatly increased, but they will suffice, I think, to convince one of the difficulty of giving a sharp definition to “species.” The facts speak strongly in favor of the transmutation theory, and show us how a species may become separated under different conditions into a number of new forms, which would be counted as new different species, if the intermediate forms were exterminated.
In discussing the nature of the changes that bring about variability, Darwin remarks: “From a remote period to the present day, under climates and circumstances as different as it is possible to conceive, organic beings of all kinds, when domesticated or cultivated, have varied. We see this with the many domestic races of quadrupeds and birds belonging to different orders, with goldfish and silkworms, with plants of many kinds, raised in various quarters of the world. In the deserts of northern Africa the date-palm has yielded thirty-eight varieties; in the fertile plains of India it is notorious how many varieties of rice and of a host of other plants exist; in a single Polynesian island, twenty-four varieties of the breadfruit, the same number of the banana, and twenty-two varieties of the arum, are cultivated by the natives. The mulberry tree of India and Europe has yielded many varieties serving as food for the silkworm; and in China sixty-three varieties of the bamboo are used for various domestic purposes. These facts, and innumerable others which could be added, indicate that a change of almost any kind in the conditions of life suffices to cause variability—different changes acting on different organisms.”