now to find the way in which birds (or other animals) may be most conveniently arranged, but to discover their pedigree, and so construct their family tree. Such a genealogical table or phylum (λῡΦον, tribe, race, stock) as it is called, is rightly considered the only sound basis of taxonomy. In attempting this end, we proceed upon the belief. . . that all birds, like all other animals and plants, are related to each other genetically, as offsprings are to parents; and that to discover their genetic relationships is to bring out their true affinities—in other words, to reconstruct the actual taxonomy of nature. In this view there can be but one 'natural' classification, to the perfecting of which all increase in our knowledge of the structure of birds infallibly and inevitably tends. The classification now in use, or coming into use, is the result of our best endeavors to accomplish this purpose, and represents what approach we have made to this end. It is one of the great corollaries of that theorem of Evolution which most naturalists are satisfied has been demonstrated. It is necessarily a—
Morphological Classification; that is, one based solely on consideration of structure or form (μοΦρή, morphe, form); and for the following reasons: Every offspring tends to take on precisely the structure or form of its parents, as its natural physical heritage; and the principle involved, or the law of heredity, would, if nothing interfered, keep the descendants perfectly true to the physical characters of their progenitors; they would breed true and be exactly alike. But counter influences are incessantly operative, in consequence of constantly varying external conditions of environment; the plasticity of organization of all creatures rendering them more or less susceptible of modification by such means, they become unlike their ancestors in various ways and to different degrees. On a large scale is thus accomplished, by natural selection and other natural agencies, just what man does in a small way in producing and maintaining different breeds of domestic animals. Obviously amidst such ceaselessly shifting scenes, degrees of likeness or unlikeness of physical structure indicate with the greatest exactitude the nearness or remoteness of organisms in kinship. Morphological characters derived from examination of structure are therefore the surest guides we can have to the blood-relationships we desire to establish; and such relationships are the 'natural afiinities' which all classification aims to discover and formulate. (Coues.)A few terms in general use may receive a moment's discussion. A type or group is said to be specialized when it has a relatively large number of peculiarities, or when some one peculiarity is carried to an extreme. A sculpin is a specialized fish, having many unusual phases of development, as is also a sword-fish, which has a highly peculiar structure of the snout. A generalized type is one with fewer peculiarities, as the herring in comparison with the sculpin. In the process of evolution, generalized types usually give place to specialized ones. Generalized types are therefore as a rule archaic types.
The terms high and low are also relative; a high type being one with varied structure and functions. Low types may be primitively generalized, as the lancelet in comparison with all other fishes, or the herring in comparison with the perch; or they may be due to degradation, a loss of structures which have been elaborately specialized in their ancestry.
