Problems of Genetics

by William Bateson

We cannot tell whether yellow or not-yellow is due to the presence of a factor, but we may suppose that one or other gives the special colour to the parts. The black of character 3 is no doubt a dominant. Thus pinus becomes Y¹y²b and chrysoptera in y¹Y²B. The Lawrencei which has the underparts yellow, wing-bars white, and black patches is Y¹y²B and leucobronchialis which has mantle and underparts not-yellow, wing-bars yellow and no black patches is y¹Y²b. This representation, it should be clearly understood, is tentative and approximate only. The characters are not really sharp, for there is much grading; but allowing for the effects of heterozygosis and for some actual breaking-up of factors I believe it gives a fairly correct view of the case. In particular we can see how it meets the difficulty which Chapman felt in accepting leucobronchialis as in any sense derived from pinus which has a yellow breast, and chrysoptera which has a black throat, seeing that leucobronchialis has neither. We now recognize at once that this form could be produced by ordinary re-combination of the absence of Y¹ with the absence of B.

I note also with great interest that the modern observers agree that the so-called hybrids may have the song either of the one species, or of the other, or a song intermediate between the two. It may also be added that these two types have several times been seen, in the breeding season, paired with each other or with one of the other combinations.

Fig. 1.	Helminthophila pinus, male.
Fig. 2.	Helminthophila pinus, female.
Fig. 3.	"Lawrence's Warbler," male; one of the integrading forms.
Fig. 4.	"Brewster's Warbler," male; another of the integrading forms.
Fig. 5.	Helminthophila chrysoptera, male.
Fig. 6.	Helminthophila chrysoptera, female.

Allen[11] has described another excellent American example, the Tits of the group Baeolophus bicolor-atricristatus. The form bicolor belongs to the eastern States and ranges from the Atlantic coast to the Great Plains, and atricristatus, of east Mexico, extends from Vera Cruz to central Texas. In southern and central Texas the breeding ranges adjoin, and in this country various intermediates occur. The chief types differ in two main points.

B. bicolor	B. atricristatus
Forehead varies from deep black	Forehead white to buffish white.
to dull black, suffused with rusty brown.
Crown and crest grey,	Crown and crest black, abruptly
slightly darker than the back.	contrasting with the back.

The intergrades between the two have, as usual, received specific names. A detailed description is given by Allen, from which it appears that the gradation is very complete. In one case a series of 16 adults were all intermediates. It is not stated whether the collector took these at random, but from the local lists it is clear that the types are found not far away from the place where the intergrades were shot.

Another very striking case is that of the Tanagers, of the genus Rhamphocoelus. In this group there are several local forms which are related to each other in remarkable ways. The forms known as passerinii and icteronotus exhibit the clearest phenomena of intergradation. The species passerinii has a brilliant scarlet and black male, and it inhabits Honduras and Nicaragua. Proceeding southwards along the isthmus we find next costaricensis which has a male like that of passerinii (but a female with more orange than the olive-grey female of passerinii). Next we come to Panama which is occupied by icteronotus, sharply distinguished from passerinii by the fact that the scarlet is replaced by lemon-yellow. This same icteronotus occurs again as a pure type in Ecuador and many other parts of South America; but Colombia, between Panama and Ecuador, contains scarlets like passerinii, yellows like icteronotus, and various intergrades of several shades of orange. The passerinii males from Nicaragua are indistinguishable from those of Colombia, and the icteronotus of Ecuador are the same as those in Panama. The orange intergrades, doubtless heterozygous forms, though collected at the same locality (Medellin in Colombia) as several pure yellows and pure scarlets, are in the British Museum series sorted out as a separate species under the name chrysonotus! Complications are introduced by the relations of these forms to another named type, flammigerus, but we may for our purpose leave that out of consideration, and say that the order of geographical sequence from Honduras to Ecuador is (1) scarlet, (2) yellow, (3) mixture of types, scarlet, yellow, orange, (4)yellow.

Similar examples exist in the birds of the old world, but I do not know of any that have been studied so fully as those of America. The best known is that of the two Rollers, Coracias indicus which spreads from Asia Minor through Persia, Baluchistan, the Indian Peninsula and Ceylon, and affinis which ranges from Nepal, through Assam, Tenasserim and the Indo-Chinese countries. The two types are very different and may be distinguished as follows: