THE PHOBLEM OF ORGANIC DEVELOPMENT. 185 



many pigeons; and if we wish to take a part in it, all we have to do is to provoke 

 its repetition at proper intervals. 



First, let us see what nature does, and then what we can add to the demonstra- 

 tion. In the common pigeons, as I have said, the apical mark is a vanishing charac- 

 ter which comes to an end with the Juvenal period of development. In the second 

 plumage no trace of it appears. There seems to be a sudden jump from one stage 

 to the next, with no intermediate transitional phases. To a mutationist it might 

 seem that ontogeny was recapitulating a final phyletic sault "a sudden strange 

 transfigurement.' ' 



If such be the nature of the passage from the Juvenal to the adult plumage, what 

 should we expect in case a number of the first feathers should begin development 

 later than the rest? Would the belated feathers appear in the Juvenal or the adult 

 pattern? Certainly it should be either one stage or the other, for the mutation 

 theory does not admit of midway transitions. 



We have just this test of the theory given regularly in a small tract of the lesser 

 coverts of the wing. Two or three rows of these coverts, forming an oblique tract 

 crossing the fore-end of the wing, from wrist to elbow, in line with the bones of the 

 forearm, appear 10 to 14 days later than the rest. 



In our common two-barred pigeons this tract of feathers always makes a nearer 

 approach to the adult color than do the other Juvenal feathers. Its gray is freer 

 from the brownish tinge characteristic of the first plumage, and the apical marks are 

 lost, or reduced to vestiges difficult to recognize (text-fig. 15). In chequered varieties 

 more distinct traces of these marks are generally visible, but it is only now and then 

 that our domestic birds give us the test in a fairly satisfactory form. A single 

 example happens to be available at this time; it is a young blackish-brown hybrid, 

 about 15 days old, from a male red barb (offspring of a pair of black barbs) and a 

 female red and white jacobin. The feathers of this hybrid have dull reddish tips 



1 mm. wide. The oblique tract, lying between the lesser and median coverts, has 



2 to 3 rows of pin-feathers, which are nearly all concealed by the overlying lesser 

 coverts. Some of these pins are just beginning to unfold, and 3 to 4, near the bend 

 of the wing, are long enough to protrude 3 to 5 mm. beyond the lesser coverts. In 

 this specimen (text-fig. 16) the feathers of the oblique tract, so far as unfolded, show 

 the apical mark, but weaker in color and reduced to about half width. We see here 

 progress halted about midway between Juvenal and adult conditions. 



In this connection I may refer to another hybrid, now a year old and in adult 

 plumage. This hybrid was obtained from a male homer (black-chequered) and a 

 female oriental turtle-dove. The homer has no apical marks; the turtle-dove has 

 the mark as a permanent character. What should we expect in the hybrid? In the 

 Juvenal feathers the mark was much stronger than in common pigeons, but weaker 

 than in the turtle-dove. In the adult feathers I find that the mark has been pre- 

 served, and at a width of about 1 mm., or nearly the maximal width seen in the 

 Juvenal feathers of common pigeons. 



These two hybrids exhibit the mark in intermediate conditions, the one in the 

 juvenal feathers, the other in both Juvenal and adult feathers. Such evidence 

 can be easily multiplied to various degrees of transition, but the test would be 

 more demonstrative if the degrees were presented in natural sequence in a single 

 individual. 



