Problems of genetics / by William Bateson.

  • Bateson, William, 1861-1926.
Date:
1913
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Problems of genetics / by William Bateson. Source: Wellcome Collection.

    173/290 (page 151)
    OVERLAPPING FORMS of the Rio Grande or the Gulf of Mexico. Between the two lines thus roughly defined is a band of country about 1,200-1,300 miles long and 300-400 miles wide, which contains some normal birds of each type, but chiefly birds exhibiting the characters of both, mixed together in various and irregular ways. Even in the areas occupied by the pure forms occasional birds are re¬ corded with more or less indication of characteristics of the other form, but within the area in which the two forms are conterminous, the mixed birds are in the majority. The condition of these birds of mixed character is described by Allen as follows : As has been long known—indeed, as shown by Baird in 1858—the 'intermediates' or 'hybrids' present ever-varying combinations of the characters of the two birds, from individuals of C. auratus presenting only the slightest traces of the char¬ acters of C. cafer, or, conversely—individuals of C. cafer present¬ ing only the slighest traces of the characters of C. auratus— to birds in which the characters of the two are about equally blended. Thus we may have C. auratus with merely a few red feathers in the black malar stripe, or with the quills merely slightly flushed with orange, or C. cafer with either merely a few black feathers in the red malar stripe, or a few red feathers at the sides of the nape, or an incipient, barely traceable scarlet nuchal crescent. Where the blending of the characters is more strongly marked, the quills may be orange-yellow or orange-red, or of any shade between yellow and red, with the other features of the two birds about equally blended. But such examples are exceptional, an unsymmetrical blending being the rule, the two sides of the same bird being often unlike. The quills of the tail, for example, may be part red and part yellow, the number of yellow or red feathers varying in different individuals, and very often in the opposite sides of the tail in the same bird. The same irregularity occurs also, but apparently less frequently, in the quills of the wings. In such cases the quills may be mostly yellow with a few red or orange quills intermixed, or red with a similar mixture of yellow. A bird may have the general coloura¬ tion of true cafer combined with a well-developed nuchal crescent, or nearly pure auratus with the red malar stripes of a cafer.