384 
Mr. R. F. Tomes on White’s Thrush. 
Having proceeded with this method as far as practicable, the 
remainder would have to be considered. Amongst such re¬ 
mainder will be found species differing too much from each 
other to be with consistency grouped together, but which never¬ 
theless are not strongly marked enough to induce us to make a 
new genus for each one. But we are compelled either to do 
this, or place them all together-—the constancy of the characters 
not being here determinable. 
Such has been pretty much the case amongst the limited 
number of Turdidce which I have had for comparison. I have 
now before me the following species— Turdus musicus, T. iliacus, 
T. ohscurus, Gm., T. pilaris , T. viscivorus , T. torquatus , T. 
cyaneus , T. merula , T . migratorius, T. mustelinus, and a few 
other species, besides Oreocincla dauma , O. aurea, and O. lunulata. 
From these I select the first three, their general similarity being 
so great, even to the distribution of the colours and the 
markings of the feathers, that the most determined genus- 
maker would scarcely attempt to sever them. But on com¬ 
paring the Song-thrush and Redwing, I find, besides the discre¬ 
pancy in size, a greater relative length of wing in the latter 
than in the former. I shall proceed to analyse the value of this 
difference, as a means of arrangement of the two species. 
The wing itself is structurally similar in both of them ; that 
is, it has the same relative length of those feathers which con¬ 
stitute the end of the wing, but they are somewhat longer in 
relation to the secondaries and tertiaries in the Redwings. This 
difference in the length of the wing, indicative of greater or less 
powers of flight, is explained by the migratory and resident 
habits of the species; and on examining those internal parts 
which are instrumental in flight, we find that, although the 
migratory species is the smaller of the two, and has a sternum 
proportional to its size, the keel is fully as deep as in the 
larger and resident species: relatively , therefore, it is deeper. 
This is just as it should be: the longer wing necessary for the 
performance of specific migratory habits requires a greater 
development of muscle to wield it; and this is accomplished by 
the greater depth of the keel. This peculiarity is a purely 
adaptive one; there is no indication of an essential character in 
