THE NATURE OF SPECIES
By John Walter Gregory
Professor of Geology, University of Glasgow
The final test of the theory of organic evolution is whether the different kinds of animals and plants are fixed and unchangeable or whether one kind may through its posterity give rise to or pass into another kind, even though the passage may be so slow that the changes produced in the lifetime of one observer are slight. No highly specialized organism can be expected to develop into an altogether different organism; there is no chance, for example, that a humble-bee will give birth to an elephant, or that a club moss will develop into a fruit tree. Nor can any decisive verdict as to the natural evolution of new forms of animals or plants be given from a consideration of the various breeds of sheep or dogs or garden plants. New breeds can unquestionably be developed at the will of the breeder; but the fact that domesticated animals can be varied by breeding does not necessarily show that under natural conditions the progeny of one kind of elephant can become another kind, or that a certain sort of moss, if placed in a new environment, will become another kind of plant.
The work done by breeders shows the plasticity of living forms, and like plasticity is seen in animals and plants living under natural conditions, but the question is whether there are in the animal and vegetable kingdoms any well-established units between which distinctions—even comparatively slight distinctions—can be marked by boundaries that are
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