Page:Popular Science Monthly Volume 63.djvu/24

20 and kinetic theories of evolution. Under the former mutations have been accepted as genuine examples of the methods by which species are formed in nature, while under the latter they appear as but the dying spasms of small groups of organisms suffering a fatal separation from the life of their species.

Mr. A. F. Woods has kindly brought to my attention an important confirmation of this association of mutation with reproductive debility, namely, that cultural methods calculated to encourage vegetative growth at the expense of reproductive vigor or fertility are also distinctly favorable to the appearance of mutations and of physiological abnormalities such as variegation of foliage. Professor De Vries made Oenothera the special object of his study because the frequency of fasciation and other monstrosities seemed to indicate a high degree of structural instability. The abnormality of this class of evolutionary phenomena was not considered. It was inferred instead that the condition of 'mutation' is a somewhat rare and temporary state through which organisms pass at the period of formation of new species, and the failure to find equal 'mutability' in other plants did not prevent the drawing of general conclusions.

As a summary of the above discussion three evolutionary conditions may be formally distinguished:

1.Prostholytic or Progressive Stage.—The prostholytic or progressive stage of evolution is found in large species of wide distribution containing abundant individuals with free intercrossing of numerous lines of descent. There is unlimited diversity or inconstancy of individual characters, and variation is indefinite and continuous in the sense that endless fluctuations and intergradations are present. The requirements of symbasis are fully met; interbreeding is normal and reproductive fertility is high.

2. Hemilytic or Retarded Stage.—The hemilytic or retarded stage of evolution is reached in species or subordinate groups of restricted distribution containing a limited number of individuals with few and closely interrelated lines of descent. Characters are nearly uniform and variation slight. The requirements of symbasis are not fully met, but the deficiency has not yet resulted in reproductive debility.

3. Catalytic or Declining Stage.—The catalytic or declining stage of evolution appears in closely segregated groups of relatively few individuals propagated by inbreeding or on single lines of descent. Variations are few, pronounced, and abrupt or discontinuous, also relatively constant and with little or no intergradation. The catalytic stage implies a violation of the law of symbasis, or inadequate cross-fertilization, together with the resulting deficiency of fertility.