Page:Popular Science Monthly Volume 68.djvu/485



ITHIN any given species of animals or plants as occurring in nature, variations of many sorts may appear. No one individual is the exact image of another, either in structure or in function. In theory, at least, no one cell is exactly like another, no one chromosome the exact duplicate of a mother or a sister chromosome. Moreover, no one group or aggregation of individuals is exactly like another, if separated from it by time or space. In the classification of variations we may naturally divide them into individual variations and collective variations. Collective variations are produced by the extension of certain types of individual variations from generation to generation. These form the basis of new species, as gaps are produced within the series by isolation or by the death of intermediate forms.

Individual variations are again sharply divided into those which are inborn or blastogenetic, and those which are acquired or ontogenetic, produced by the direct influence of environment or by the reaction of the organism from external conditions.

The inborn variations arising from differences in the original germ cells, male, female or both, or from results of their combination or amphimixis, may be again subdivided as fluctuations, saltations, monstrosities and hybridizations. So far as we know it is alone from inborn variations, as perpetuated by heredity, as sifted by natural selection and as protected by bionomic isolation, that collective variations, nascent species and new species originate.

The small differences, numerous but slight, and never wanting, which distinguish one individual from another of the same species