Page:Popular Science Monthly Volume 68.djvu/499

Rh the characters of a species are not those which appear, but the ability to develop these characters under the conditions surrounding the ancestry of the individual.

But there is no evidence that the direct influence of environment is a factor in the separation of species, except as its results may be acted upon by natural selection. We have no reason to suppose that the environment of one generation determines the heredity of the next. It is true in a broad way that the ill-nourished offspring has weaker or less numerous offspring, but weak or strong, their hereditary traits are those of their actual parent stock.

The features of the 'ontogenetic species' or subspecies, have long been known under the name of 'convergence of characters,' 'parallelism' and 'analogous variation.' An 'ontogenetic species' is a group in which the likeness among the members is due, not to genetic connection, but to the exposure of the individuals to like conditions of development. Hence it should have no recognition in taxonomy, which deals with phylogenetic species and subspecies only. But no species is truly defined when only the usual characters, those developed under usual conditions, are considered. To know the species fully, we should know what qualities individuals may develop under all the varied relations of the environment in which they may be placed.

Ontogenetic species, however, tend to become phylogenetic, in isolation from the rest of their kind, by interbreeding among themselves, and under new conditions of selection. The real characters of the race thus formed may be wholly obscured by the more evident characters due to food conditions or to reaction from the environment. To test the characters, phylogenetic and ontogenetic, and to purge the system of species and subspecies founded on the latter, will be part of the work of the student of species for a long time to come.

Taking concrete illustrations—the Loch Leven trout, Salmo levenensis, recently discussed, is distinguished in its native waters by certain obvious characters. These disappear when the eggs are planted in brooks in England or in California, and the species develops as the common English brook trout. But it is conceivable that the obvious or ontogenetic traits of the Loch Leven trout are not the real or phylogenetic distinctions, and that the latter, more subtle, engendered through individual variation, inheritance, selection and isolation, really exist, although they have escaped the attention of ichthyologists.

After the species was planted in Yosemite Park in 1896, it remained for nine years unnoticed. In 1905, individuals sent to me were, so far as I could see, exactly like English brook trout. But it is conceivable that differences in food and water have caused slight ontogenetic distinctions. It is certain that in isolation from all parent stocks they will, in time, develop larger differences, which, after many thousand generations, will be specific or subspecific. At present, these