THE REALITY OF BIRD SPECIES.
BY LEVERETT MILLS LOOMIS.
In 1858, in volume IX of the ‘Reports of Explorations and Surveys ... from the Mississippi River to the Pacific Ocean,’Ammodromus samuelisBaird andMelospiza fallaxBaird appear as full-fledged species. In 1874, in ‘A History of North American Birds,’ Land Birds, volume II, these so-called species are reduced in rank, being designated respectivelyMelospiza melodia, var.samuelis, Baird andMelospiza melodia, var.fallax, Baird. In 1886, in the first edition of the A. O. U. ‘Check-List,’ these names are altered, in accordance with earlier lists by Mr. Ridgway and Dr. Coues, toMelospiza fasciata samuelis(Baird) andMelospiza fasciata fallax(Baird), pure trinomials and the term subspecies having come into vogue. In 1910, in the third edition of the A. O. U. ‘Check-List,’ the two names are amended toMelospiza melodia samuelis(Baird) andMelospiza melodia fallax(Baird).
Owing to his lack of knowledge of geographic variation, Professor Baird gave to each of these geographic variations of the Song Sparrow an entity which they did not possess, and this entity, having gained a foothold in the literature, is perpetuated today in the subspecies (‘incipient species’). As no one can foresee the future of these variations of the Song Sparrow, it is not known whether they are the beginnings of species or not. Nevertheless, it may be urged that birdhistory repeats itself, and that the record of past events warrants the conclusion that bird species are now in process of evolution through geographic variation. Theorize as we may, the fact remains that we do not know what part geographic variation or other agencies played, or did not play, in the origin of existing bird species, themodus operandiof the evolution being unknown. But we do know that geographic variation is one of the common variations occurring within the bounds of a bird species of today, and that it is not the only variation in which geography is a factor.
Independent of individualism, age, sex, season, or climatic conditions, there exists a type of variation known as dichromatism, which perhaps originated in mutations. It is well exemplified in the Jaegers, Albatrosses and Petrels, Herons, Hawks, and Owls. In some species there is a difference in the geographic range of the phases, but it does not correlate with environment as in geographic variation. Instances to the point are found in the Wedge-tailed Shearwater, Red-tailed Hawk, and Screech Owl.
More than thirty years ago, when our knowledge of variation was far less than it is now, Dr. Stejneger had the discernment to interpretColaptes auratus(Linnæus),Colaptes cafer(Gmelin), andColaptes hybridus(Baird) to be dichromatic or trichromatic phases of one species, and not two species that hybridize on a gigantic scale.[21]None of the characteristics of dichromatism are wanting in these extremes and intermediates. They are similar in general character to the extremes and intermediates of well-known dichromatic species, of the Wedge-tailed Shearwater, Neglected Petrel, and Rough-legged Hawk for example. They are not individual and are not dependent upon age, sex, season, or environmental conditions. Moreover, intermediates crop out sporadically in the Eastern States, where theauratusphase is dominant. It is well to bear in mind that these variations of the Flicker are not greater than certain other normal variations; as the age variation of the Western Gull, the sexual variation of Williamson’s Sapsucker, the seasonal variation of the Marbled Murrelet, and the dichromatic variation of the Parasitic Jaeger.
The question naturally arises, whether dichromatism has often been misinterpreted and made the basis of apocryphal species and their supposed hybridization on a grand scale. In the alleged Junco species, for instance, possibly dichromatism or polychromatism, originating in mutations, obtains along with geographic variation.
Vermivora leucobronchialis(Brewster) andVermivora lawrencei(Herrick) are not overlooked in this discussion. The evidence thus far presented tends to prove that they are hybrids between two species rather than intermediates of one dichromatic species.[22]Be this as it may, hybridization between unquestionable species of birds is an abnormal and relatively rare occurrence.
To affirm that bird species are concepts, is to ignore the facts in the case.Ammodromus samuelisBaird andMelospiza fallaxBaird are concepts, butMelospiza melodiawith all its geographic variations is a reality. It is absolutely separated fromMelospiza lincolniandMelospiza georgianaand all other existing bird species.Colaptes auratusis likewise a reality. In spite of its great dichromatic variation, it does not intergrade with any other woodpecker. It is confidently stated that the great majority of the A. O. U. ‘Check-List’ species are also realities, and the remainder time-honored concepts based on inconstant variations, likeFulmarus rodgersiCassin, which is merely an extreme white phase ofFulmarus glacialis(Linnæus).[23]
In a word, absence of intergradation among birds results in a definite entity, the existing bird species.