250 Mr. W. P. Pycraft on the 
muscles, for example, reveals nothing, so far as the question 
of relationship between the two groups is concerned ; 
inasmuch as the Cypseli have undergone very profound 
modifications with regard to the wing—a degree of extreme 
specialization which has obliterated all the more normal, 
more primitive characters. 
In the muscles of the thigh the Cypseli shew no less 
extreme modifications, or, as Dr. Chalmers Mitchell has it, 
apocentricity. 
The plantar tendons of these two groups appear, at first 
sight, to be no more helpful in this connexion, inasmuch as, 
so far as the matter has been investigated, three very distinct 
combinations of these tendons have been brought to light, 
combinations represented respectively by the Colies, Swifts, 
and Humming-birds. If correctly interpreted they shew, 
however, that they must be regarded as so many modifications 
of a common and primitive plan, wherein the flexor longus 
hallucis split up to serve each of the four toes, while the 
flexor perforans digitorum was restricted in its ramifications 
to digits II., III., [V., the two tendons being unconnected by 
avinculum. Such an arrangement, among lying birds, has 
been found, so far, only in Heliornis, an aberrant Gruine bird. 
It would be beside the purpose of the present paper to enter 
into a discussion of the evolution of all the known types 
of plantar tendons, but we may gain a most instructive 
insight into the matter by the tendons in the Colies and the 
forms therewith associated—the Swifts and Humming-birds. 
Curiously enough, the Humming-birds approach nearest 
to this primitive type, though even there a considerable 
degree of specialization has continued to mask the evidence 
of the earlier condition of things *. 
* Before proceeding further it would be well to remark that the 
description and figure of the plantar tendons of the Humming-birds 
given in Prof. Newton’s ‘ Dictionary of Birds’ by Dr. Gadow are in- 
correct. This fact was pointed out by Mr. F. A. Lucas in ‘The Ibis,’ 
1895 (8), and appended to this correction will be found an acknow- 
ledgment by Dr. Gadow (5) of the accuracy of Mr. Lucas’s remarks and 
the figure accompanying them, I have now, by dissection, been able 
to further confirm Mr, Lucas. 
