162 ORTHOGENETIC EVOLUTION IN PIGEONS. 



If a visible character varies it is not accounted directly responsible, for the 

 change is referred to a "premutation" accomplished in the corresponding primordial 

 pangen. Thus the visible appearance and the initial, responsible change are at 

 least as far apart as the origin of the germ and that stage in its development when 

 the variation first manifests itself; and in the case of a latent pangen they may be 

 many generations, or even ages, apart. Through all these generations and ages, 

 the pangen must be steered onward and finally delivered at the exact time and place 

 of destination, not by autonomous migration, but by the aid of a distributing ap- 

 paratus that must arise de novo at every transfer-station. The old pre-formation 

 theory of Bonnet settled all at a single miraculous creation; but here we have self- 

 propagating miracles ad infinitum. 



Note. — With these introductory remarks, the lecturer turned to the consideration of 

 a few specific characters which are found in various species of pigeons. 



The wing-bars found in domestic races and in the wild rock-pigeons were taken as an 

 example to illustrate orthogenetic evolution by gradual progressive modification. It was 

 shown first of all that the two-barred condition seen in the typical Columba livia is derived 

 from the chequered condition seen in the wild C. affinis. It was then shown that this 

 mode of derivation is widespread among wild species of pigeons, the bars always resulting 

 from a reduction of the chequers, proceeding from before backward, the direction being 

 the same as that of embryonic development. 



Experiments with domestic pigeons demonstrate that it is easy to reduce the chequered, 

 type to 4 bars, then to 3, 2, 1, and finally to a uniform gray color without a single bar. 

 Another set of experiments, to test the possibility of reversing the process, by advancing 

 from the two-barred condition to the uniformly chequered type, showed that the direction 

 could not be reversed. 



It was next shown that among wild species of pigeons we have the same law of ortho- 

 genesis illustrated over and over again, in almost endless variety of conditions. The 

 wild passenger-pigeon, the mourning-dove, the zenaida, the ground-dove of Florida, and 

 a number of Old World species were shown in colored drawings and on charts. 



After tracing wing-bars of the most diverse kinds to chequers, the origin of the latter 

 from a still earlier and umversal avian character was explained. This earlier color-mark 

 still persists in many pigeons and other avian types, and is well preserved in the oriental 

 turtle-dove of Japan and China. It consists of a single dark spot occupying the center 

 of the exposed part of each feather. In the course of evolution this spot has been divided 

 into two lateral spots by the disappearance of pigment along the shaft, beginning at the 

 apex of the feather and advancing gradually inward. The old turtle-dove character thus 

 passes by a continuous process of division into the rock-pigeon pattern, consisting of two 

 chequers on each feather, more or less completely separated. The evidences showing such 

 a gradual transmutation are still to be seen, and in such profusion as to wholly exclude 

 doubt. Hundreds of species have been formed in this simple way, leaving no room for 

 the claims of sudden, non-transitional mutations. 



The transitional stages between the turtle-dove pattern and the chequered pattern 

 of the rock-pigeons are exhibited not only as we pass from one species to another, but often 

 as we advance from the juvenal to the adult plumage; and frequently they may be seen 

 in different parts of one and the same individual plumage. 



This passage from the juvenal to the adult plumage often presents us with two or more 

 quite strongly contrasted color-patterns. The mutationist might see in this a striking 

 illustration of discontinuity in the evolution of species. In the normal course of events 

 the gap is often very wide between two successive patterns, but closer study and experi- 



