" QUINTOCUBITALISM " IN THE WING OF BIRDS. 231 



important to the future bird, and early provision must be made 

 for them. 



I suggest that the quills, although appearing to be a horizontal 

 row, are really different members of diagonal transverse series. 



There now remains to suggest an explanation of the origin of 

 the gap. In a general way, morphologists have been inclined to 

 regard simple series, as in the case of metameric repetitions, as 

 more primitive than discrete and incoherent series; but in a 

 number of cases, as for instance in the nephridia of earthworms, 

 it would appear that the diffuse condition is simpler and more 

 primitive than the orderly repetition found in the common 

 earthworm. I think we have learned to be on our guard against 

 taking for granted that au apparently simpler condition is in 

 reality more ancestral, and that we should approach the problem 

 of the diastataxic gap without any prejudice in favour of its 

 being secondary. Whether it be secondary or primitive, we have 

 to account not only for its existence but for its appearance in a 

 definite place after five quills. If the primitive condition were 

 eutaxic and the gap produced in a whole series of different birds 

 by secondary lengthening of the wing, we should have to explain 

 why this lengthening always occurred after five quills. If we 

 think of the gap as being primitive, there is no difficulty what- 

 ever in supposing that convergent closing should have occurred 

 independently in any number of groups, or even of species and 

 genera ; and I think it is not difficult to form some idea as to 

 how a gap in that position might have come into existence in 

 an ancestral wing. Consider an ancestral pentadactyle wing 

 provided with scales or scale-like feathers. The most common 

 arrangement of these, as may be seen by looking at the scales 

 on any lizard or crocodile or on the feet of birds, is that longi- 

 tudinal rows should run along the digits and diagonal transverse 

 rows should surround the arm. Now these two series have 

 to meet somewhere, and when different series of scales or 

 markings meet, there must be a transition of some kind from 

 one to the other. Sometimes one set of series gives way to the 

 other ; sometimes there is intercalated between the two a wedge- 

 shaped set of rows, as Professor D'Arcy Thompson pointed out 

 to me in the case of the markings on the zebra. In the foot of 

 a bird (fig. 7, A) the transverse rows are represented by largd 

 single scutes in many cases, while a line of enlarged scutes may 



