that connect the Orders and Families of Birds. 429 
Merops becoming shorter as they approach Hirundo ; while those 
of some of the latter group partially desert their own type, and 
. by degrees assume the lengthened form of the bill of the Bee- 
Eaters. The tail of Merops, again, is equally found to desert the 
typical character of the group, namely, the greater length of the 
two middle feathers, in order to become even in some species, 
then slightly forked, and at length to be identified with the fully- 
forked tail of Hirundo. 
In continuing to trace out the same line of affinities between 
the succeeding groups, it is unnecessary to enter into a detail of 
those characters which unite the two families of Caprimulgide 
and Hirundinide. 'The union between them in the most essen- 
tial particulars, in their habits, economy, and general conforma- 
tion, is too evident to the common observer, and too universally 
acknowledged by scientific writers, to need any further illustra- 
tion. It is gratifying, however, to observe, how even in mi- 
nute particulars a gradual succession of aflinities imperceptibly 
smooths the passage between conterminous groups; and I can- 
not pass over without remark, the circumstance of the hind toe of 
Caprimulgus being usually retractile, which enables it to place all 
its toes in front, in a similar position to that which they maintain 
in Cypselus, where the family of the Hirundinide terminates. The 
conformation of the tail in the two families shows a similar affi- 
nity. Some species of Caprimulgus lately arrived from Brazil, 
exhibit the forked tail of Hirundo, one of which indeed, the 
C. psalurus of M. Temminck, has this character developed to an 
almost disproportionate degree. 
Leaving those typical families with the short bill, and taking a 
general survey of the tribe, we may perceive that the Caprimul- 
gide unite themselves to the longer-billed families by means of 
the Linnean Todi, which preserve the broad base of the bill of the 
latter, but lead on by the comparative length of that member to 
the succeeding family of Halcyonide. If we compare the bill of 
3x2 the 
