384 Mr. R. F. Tomes m 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 Turdicl(E which I have had for comparison. I have 

 now before me the following species — Turdus musicus, T. iliacus, 

 T. obscurus, Gm., T. pilaris, T. viscivorus, T. torquatus, T. 

 cyaneus, T. meriila, T. migratorius, T. mustelinus, and a few 

 other species, besides Oreocincla dauma, O. aurea, and O. lunulata. 

 From these 1 select the first three, their general similarity being 

 so great, even to the distribution of the colours and J;he 

 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 



