2 2 POPULAR SCIENCE MONTHLY. 



ing of closely related individuals, as distinguished from fertilization by 

 the pollen of the same flower, and since domestication implies inbreed- 

 ing the habit of self-fertilization would involve no additional injury, 

 but would have an important practical advantage in greatly increasing 

 the chances of pollination and seed-production. 



Mutations and Hybrids. 

 The recognition of symbasis, or the necessity of a broad foundation 

 to sustain the organic structure, permits the inference that some hybrids 

 are sterile and variable for the same reason that closely inbred plants 

 and animals decline in fertility and produce mutations or deviations 

 from the normal type. A hybrid is a mixture or cross between indi- 

 viduals which would not be expected to mix in nature. Among domes- 

 ticated plants hybridization signifies the reverse of selection, the cross- 

 ing of varieties which the breeder commonly strives to keep separate. 

 Generalizations to the effect that hybrids as a whole are sterile, variable, 

 weak or vigorous are fallacious, since the results of the crossing depend 

 upon the evolutionary status of the parents. By segregation or in- 

 breeding normal or progressive variation gradually gives place to uni- 

 formity and then to mutation, but hybrids between distant types pass 

 at once from the progressive stage to the catalytic. On the other 

 hand, crosses between inbred or closely segregated stocks may show 

 increased vigor and stability, and thus reverse the process of decline. 

 Hybrids, therefore, may be either prostholytic or catalytic as they tend 

 upward or downward in the evolutionary series. 



Diagram of Evolutionary Stages. 

 Sterility. 

 Catalytic or Aberrant or mutative hybrids. 



declining stage. 

 Dialytic or Mendelian hybrids, characters an- 



divergent stage.* tagonistic. 

 Symbasis. Prostholytic or ' Inconstancy ' with intergrada- 



progressive stage. tions, as in natural species. 



Hemilytic or Uniformity or ' fixed' types. 



retarded stage. 

 Catalytic or De Vriesian mutations or ' sports.' 



declining stage. 

 .Sterility. 



Cross-breeding and close-breeding have the same limits of sterility; 

 and between each and the mean of normal evolution there is, as shown 

 by the experiments of Mendel, Garton, De Vries and others, a region 

 of the relatively infertile abrupt variations variously termed sports, 



* The dialytic or divergent stage might be described as the reverse of the 

 hemilytic; it is characterized by the facts discovered by Mendel, Spillman and 

 others, which may be taken to signify that the characters upon which close-bred 

 varieties have diverged do not combine into an average in the hybrid offspring, 

 but remain antagonistic and follow one or the other of the parental lines. 



Cross-breeding. 



Inbreeding. 



