THE PROBLEM OF ORGANIC DEVELOPMENT. 



187 



the upper pin, comes a feather (1 a) with the apical mark reduced to two-thirds 

 width. At the bend of the wing, overlying the outej pin, are 2 lesser coverts having 

 the mark at full width. 



Comparing the feathers in front of the four rows of coverts (i to iv) as to width 

 of the pale tip, and taking them in the order of their development, the relation 

 may be expressed as shown in the accompanying table: 



Text-figure 31. 



Same as text-figure 30, age 17 days; drawn by Hayashi, July 31, 1899. x 2. 



The fourth row is here covered up by later narrow-tipped feathers, and 5 new black-tipped feathe rs are coning 

 in. Some later feathers are among the scapulars. Arrangement: 1-1 1 1 are first to third rows of coverts. IV shows 

 fourth row of coverts — covered to near tip by the feathers of the oblique tract. 1-3 are two or three unevenly developed 

 rows of the oblique tract, la-16, see text, s is early juvenal; s', later Juvenal; s", post juvenal feathers; tertialfi 

 are indicated /. 



This stage brings out the interesting fact that the width of the pale apical mark 

 varies according to the age of the feathers. The lesser coverts in front of and the 

 four rows of coverts behind the oblique tract were the first to appear, and they all 

 have the apical mark at full width. The 13 feathers of the oblique tract are belated 

 to four different degrees, the 5 black-tipped feathers being the youngest, the rest 

 older in proportion to the width of the pale tips. We have, then, in this narrow 

 tract, the pale tips at three ages. The mark has gradually sunk from a width of 

 1.5 mm. to zero, and then the black-tip has appeared. 



Is the black a sudden apparition? It might seem so, and yet its gradual rise is 

 as certain as the gradual loss of the pale tip. We only have to look a little more 

 closely in order to see that the black does not first appear after the pale tip has 



