Page:Popular Science Monthly Volume 63.djvu/25

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These stages or states of evolution are distinguished and named in the belief that they will afford a useful addition to our evolutionary vocabulary. They are, however, parts of a connected series of events with no lines of separation between them. All organisms which too close segregation has brought to the catalytic stage have passed through the hemilytic. For example, the recently domesticated pecan tree of our southwestern states is still in the first or normal stage of evolution, and only a small proportion of the seedlings produce nuts like those of the parent tree. Selective inbreeding for a few generations would first produce uniformity, or 'fix the type,' as the expression is, by inducing the hemilytic or retarded stage of evolution, while a too narrow and persistent selection or the segregation of a single line of descent would hasten the decline and eventual destruction of the very type it might be designed to perpetuate. Coffee has not been domesticated for much more than a thousand years, and although selection has not been practised, very pronounced and constant variations are now appearing in considerable numbers, but all less fertile than the parent stock. That inbreeding tends to 'fixity' of characters is true only for a time; organisms in the catalytic stage are rendered less uniform as well as less fertile by continued inbreeding. Uniformity and vigor can be restored, as breeders already know, only by the repetition of the process of selective segregation after cross-breeding with another stock.

The catalytic stage is attained more slowly by asexual propagation, and the variability is far less pronounced, but partial or complete sterility has appeared in a considerable series of unrelated tropical plants long propagated only by cuttings, such as the banana, pine-apple, sugar-cane, sweet-potato, Irish potato, taro and yam.

Parthenogenesis may also be viewed as a form of asexual propagation, and habitual self-fertilization is another stage of sexual and evolutionary decline. Self-fertilization is supposed to be normal in several of the cereal grasses and in many other plants, though it is obviously unsafe to infer that cross-fertilization is entirely superfluous because frequently absent. With the cereals and other plants of similar history self-fertilization may prove to be a result of cultivation in northern latitudes where the weather is often unfavorable for pollination by the wind or by insects, so that selection would encourage variations least dependent upon cross-pollination. I learn from Mr. Jesse B. Norton that the more primitive, hardy, and disease-resistant oat varieties of South Europe open their glumes widely and thus invite cross-fertilization, while in most of the varieties bred in the colder and more rainy climate of Northern Europe the glumes separate much less, and do not expose the stigmas, thus showing that cross-fertilization has been abandoned. Darwin proved that there is no benefit in the