Page:Popular Science Monthly Volume 68.djvu/495

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What the significance of this extraordinary condition may be, is entirely unknown, nor do we know the determinant causes of dichromatism. It is sufficient for the purposes of this paper to refer to the fact.

A vast range of variations are ontogenetic, or dependent on influences affecting directly the life of the individual. These are not hereditary in the judgment of the present writer, and therefore they are not direct factors in the formation of species. Many investigators take a different view, believing in the direct inheritance of acquired characters, or of the effects of environmental or functional conditions, the familiar tenets of 'Neo-Lamarckism' or progressive heredity.

These ontogenetic variations are, strictly speaking, individual, appearing as collective only when many individuals have been subjected to the same conditions. They may be divided into environmental variations and functional variations, two categories which can not always be clearly separated, as variations due to food conditions partake of the nature of both.

In the epoch-making paper quoted above, and in other publications, Dr. Allen shows that climatic influences affect the averages in measurements and in color among birds. For example, in several species of birds, the total length is greater in specimens from the north, while the bills and toes are actually longer in southern specimens. That this condition is due to the influence of climate on development is shown by the fact that numerous species are affected in the same way. It is noticed also that specimens from the northeast and the northwest of the United States are darker in color than those from the interior, and again that red shades are more common in the arid southwest. Similar effects have been recently shown by a study of species of wasps. They may be produced at will by subjecting the larvæ and pupæ of