IT is a matter of common knowledge that the young of birds are ushered into the world in very different degrees of development, according to the species to which they belong. The helplessness of the callow young of the crow-tribe, for example, stands in strong contrast to the activity displayed by the young of the game-birds. Again the young of birds are remarkable for the very wide degrees of variation which obtain in the matter of clothing on their escape from the shell, variations which range from absolute nakedness to abundant feathering, albeit feathering of a peculiar type.
Out of these commonplace facts the systematic ornithologists, on the one hand, and the philosophical zoologists, on the other, have woven theories which have undergone many changes; but, so far, we venture to think, all have missed the point. A survey of the work of the systematic ornithologist will show that on more than one occasion the condition of the young at birth has been made either the corner-stone of a classification, or one of the main supports thereof.
For the one purpose or the other these young have been duly labeled and classified. In consequence, they may be contemplated from two different points of view: (1) According to their helplessness or otherwise, and (2) according as they are clothed or otherwise. When the young emerge from the shell in a fully active state, they are known as nidifugous or præcocial; those, on the contrary, which are quite blind and helpless on leaving the shell are known as nidicolous or altricial young. The nidicolous young may be, as we have already remarked, absolutely naked, in which case they are said to be ptilopædic. If, on the other hand, they are clothed, they are said to be ptilopædic. All nidifugous young are ptilopædic. The nidicolous young, being helpless and often blind, are assiduously fed by the parents, whilst among the nidifugous types, the young either feed themselves under the guidance of their parents, or accompany them in the search for food, and are fed by the way as the food is procured.
Two very different standards of thought have inspired those who selected the condition of the young at birth as a basis of avian classification. The older naturalists, adopting what we now regard as the purely artificial standards of their time, grouped birds according as