LECTURE II ARTIFICIAL SELECTION AND NATURAL SELECTION I NOW propose to consider the law to which Darwin and Wallace were led, the evidence upon which it is founded, and the conclusions which follow from it. In the method of attack I propose to follow Darwin, and I would warn you against almost inevitable disappointment, for it is with commonplace things and facts of every-day occurrence that a great theory has to deal. ARTIFICIAL SELECTION. DOMESTIC PIGEONS. - Darwin early in his inquiry felt the importance of having individual animals under close observation, so that all conditions influencing them could be determined. For this purpose domestic animals were far more suitable than wild ones, and pigeons were selected for special study for these reasons :-(1) The evidence of their descent from a common ancestor is clear; (2) Their historical records extend back many centuries; (3) Their variations are very great, all kinds being easily kept in captivity and all breeding true. There are probably at least 200 kinds of pigeons known which breed true, and these differ constantly from each other. The chief varieties are the following. (See Fig. 1.) The Pouter is a large and upright bird with a long body and long legs, a moderate-sized beak, and a very large crop and œsophagus. It has the habit of inflating its crop, producing a "truly astonishing appearance," being then "puffed up with wind and pride." The Carrier is a large bird with a very long beak. The skin round the eyes, over the nostrils, and on the lower jaw is much swollen, forming a prominent wattle. The Barb has a short and broad beak, and a wattle of moderate size. The Fantail has tail feathers to the number of 34 or even 42, twelve being the normal number. The tail is expanded and held erect. It has a peculiar gait, and a curious habit of trembling by convulsive movements of the neck. In a good specimen the tail should be long enough to touch the head. The Turbit has a frill formed by divergent feathers along the front of the neck and breast. The beak is very short. The Tumbler has a small body and short beak. During flight it has the habit of turning involuntary back somersaults. The Jacobin has long wings and tail and a moderately short beak. It has a hood formed by the feathers of the neck. The Trumpeter has a tuft of of the beak, curling forwards. feathers at the base The feet are much feathered. The coo is very peculiar, and unlike that of any other pigeon, being rapidly repeated and continued for several minutes. Among these forms there is thus great diversity in both form and colour. This diversity also affects the internal structure, for example the skull: the caudal and sacral vertebræ and also the ribs vary in number. The number of primary wing and tail feathers, the shape and size of the eggs, the manner of flight, and almost all other characters, also differ. If these birds were now found in a wild state, they would be considered to constitute distinct genera, yet they are known to be all descended from Columba livia, the blue rock-pigeon of Europe, Africa, India, &c. The arguments brought forward by Darwin to prove this are as follows : (i.) All domestic races are highly social, and none of them habitually build or roost in trees; hence it is in the highest degree probable that their ancestor was a social bird nesting on rocks. (ii.) Only five or six wild species have these habits, and nearly all these but Columba livia can be ruled out at once. a (iii.) Columba livia has vast range of distribution-from Norway to the Mediterranean, from Madeira to Abyssinia, and from India to Japan. It is very variable in plumage and very easily tamed. It is identical with the ordinary dove-cot pigeon, and except in colour practically identical with toy pigeons generally. (iv.) There is no trace of domestic pigeons in the feral condition. ! (v.) All races of domestic pigeons are perfectly fertile when crossed, and their mongrel offspring are also fertile. Hybrids between even closely allied species of pigeons are, on the other hand, sterile. (vi.) All domestic pigeons have a remarkable tendency to revert in minute details of colouring to the blue rock-pigeon. This is of a slate-blue colour, with two bars on the wings, and a black bar near the end of the tail. The outer webs of the outer tailfeathers are edged with white: these markings are not seen together in any other species of the family. This tendency to revert was demonstrated by Darwin as follows: He first crossed a white fantail with a black barb; then a black barb with a red spot (a white bird with a red tail and a red spot on the forehead). He then succeeded in crossing the mongrel barb-fantail with the mongrel barb-spot, and the birds produced were blue, with markings on the tail and wings exactly like those of the ancestral rock-pigeon. Thus two black barbs, a red spot, and a white fantail, produced as grandchildren birds having every characteristic of Columba livia, including markings found in no other wild pigeon. BARB BLACK FANTAIL WHITE BLUE ROCK BARB BLACK SPOT RED |