Page:Popular Science Monthly Volume 63.djvu/27

Rh mutations and hybrids. The weakness and sterility of too distant crosses and of too closely isolated or inbred plants or animals may be due alike to a deficiency of normal fertilization, and may be accepted as evidence that the true course of evolution lies along neither of these extremes but follows the natural mean between them.

Catalytic variations have not the indefinite number and diversity of the progressive stage; like the symptoms of other disorders of plants and animals the same or closely similar mutations recur in somewhat definite proportions, and are not peculiar to single species, but many members of a genus or family may be similarly affected. It is therefore not necessary to interpret the independent repetition of the same symptom of evolutionary debility as an evidence of the inheritance of definite character complexes or units. The truly admirable but often misinterpreted experiments of Mendel did not result in the discovery of 'principles of heredity' so much as they revealed limits of hybridization, in that hybrid plants were found which inherited the characters of only one parent. The failure of strongly divergent or antagonistic characters to combine into a permanent average in hybrids gives, however, no basis for denying that normal evolution proceeds by the synthesis or accumulation of acceptable variations, nor is abrupt or discontinuous variation in individuals in any way incompatible with the probability that in nature evolution goes forward only through the gradual transformation and subdivision of species. The emphasis placed by Bateson, De Vries and others upon abrupt variations is warranted by no general pertinence of the facts, and is but a consequence of the failure to perceive that the origination or multiplication of species is an incident rather than an instance of evolution.

Organic succession will not persist on too narrow lines of descent, does not normally leap aside from its course, and will not bridge over too broad a chasm of evolutionary divergence. Amount of difference in the external characters of two groups affords little indication regarding the behavior of their hybrids. Some groups treated by the systematists as closely related species will not even hybridize, while in other instances plants assigned to different genera are mutually fertile. Such discrepancies are doubtless due partly to inadequate classification and partly to the fact that organic evolution is attended also by a cytological or cellular evolution the progress of which may not be