ing of closely related individuals, as distinguished from fertilization by the pollen of the same flower, and since domestication implies inbreeding 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 individuals which would not be expected to mix in nature. Among domesticated plants hybridization signifies the reverse of selection, the crossing 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 inbreeding normal or progressive variation gradually gives place to uniformity 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.
| Cross-breading. | Sterility. | ||
| Catalytic or declining stage. | Aberrant and mutative hybrids. | ||
| Dialytic or divergent stage[1]. | Mendelian hybrids, characters antagonistic. | ||
| Symbasis. | Prostholytic or progressive stage. |
'Inconstancy' with intergradations, as in natural species. | |
| Inbreeding. | Hemylitic or retarded stage. | Uniformity or 'fixed' types. | |
| Catalytic or declining stage. | De Vriesian mutations or 'sports'. | ||
| 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.
