252 Mr. W. P. Pycraft on the 
strands of tendons to D. II. & III. we should have the same 
arrangement as that which obtains in the Humming-birds, 
except that the strongest strand would be that to D. IV. 
The peculiar arrangement of these tendons in the remainder 
of the Swifts appears to have been derived by a further 
modification of the plan seen in Macropteryx (text-fig. 23, 
p- 251). Thus, from the tarsal region downwards the two 
tendons have fused, while at the base of the toes a further 
change has taken place. The flewor perforans branch to D. IV. 
as seen in Macropteryx has disappeared, so that this tendon 
now supplies only D. II.-III., while D. I. & IV. are supplied 
by the fl. longus hallucis. Thus it is clear that the plantar 
tendons of Macropteryz, or rather this particular method of 
arrangement, may well have given rise to the system of 
fused tendons which obtains among the rest of the Swifts. 
A reference to text-fig. 24, p. 251, will shew at a glance how 
easily the transition from the one to the other is made, for the 
plantar tendons of Cypselus differ from those of Macropteryx 
only in that they have become fused from the ankle-joint 
downwards, while the tendon of D. IV. of the fl. perforans 
has become replaced by the tendon from the fl. long. hall. 
We now come to the Colies. Here, as we have already 
shown (p. 237), fl. longus hallucis runs to D. I.-IL., fl.perf.dig. 
to D. III.-IV. (text-fig. 22). This arrangement may weli have 
been derived by a modification resembling that found 
in the Swifts (text-figs. 23, 24), a strong connecting-band 
from the tendon to the hallux—the relic of branch II. of 
the originally complete fl. long. hallucis—grafting itself on 
to the tendon of the jl. perforans to D. I1., and ultimately 
causing the severance of this branch with its original stem, 
and its incorporation with the fl. long. hallucis, this connecting 
band answering to that shown in text-fig. 23 running to D.IV. 
In other words, as Cypselus has preserved tendons I. & IV. 
of the original complete set, so Colius has kept I. & II. 
Thus, then, the apparent differences in the plan of the 
plantar tendons of the Colies, Swifts, and Humming-birds 
are all, in reality, but modifications of a common type. 
This type is sufficiently distinct from all other known types 
