356 MESSES. A. AND E. NEWTON ON THE OSTEOLOGY OF THE SOLITAIEE. 
the case ; and one naturally turns to seek other instances in which a species differs from 
most of its allies in a similar manner. Two, if not more, such instances will at once 
occur to every ornithologist. They are those of the Capercally ( Tetrao urogallus ) 
and the Great Bustard {Otis tarda). In both these birds, I believe, the disproportion in 
size between the sexes is not greater than in the remains of the species now before 
the Society. But then another idea is called up : Tetrao urogallus departs from the 
usual habits of the Tetraonidoe by being polygamous ; and (though I am aware that the 
statement has been disputed) the same is said to be the case with Otis tarda. Indeed I 
am inclined to suppose that with all species of birds the practice of polygamy is accom- 
panied with more or less disproportion in the size of the sexes. Whether the converse 
is true I am not in a position to declare.” — Proc. Zool. Soc. 1865, pp. 716, 717. 
The strongest objection that seems possible to the suggestion here thrown out is in 
the fact recorded by Leguat, that the male birds of the Solitaire of Rodriguez assisted 
the females in the work of incubation ; but the question of its polygamy or monogamy is 
not likely to meet with a solution now. 
From the amount of variability which every portion of the skeleton of this species 
displays, it may not unreasonably be supposed that as much was exhibited in those parts 
of its structure which have perished, even if we had not Leguat’s direct evidence as to the 
individual difference in plumage among the female birds. There would accordingly have 
been abundant room for the operation of any such process as that which has been de- 
scribed under the names of “Natural Selection” and “Survival of the Fittest” to have 
operated ; and it may therefore appear at first sight, there having been known to us only 
one species of Pezophaps, that an argument against the existence of such a process might 
hereon be founded. A little reflection will, we think, show that such an argument would 
be unsound. Confined in a space so restricted as one small island, every individual of 
the species must have lived, moved, and had its being under conditions essentially iden- 
tical in all cases. Whatever power such a process might have had of operating, there 
would be neither occasion nor opportunity for it so long as no change took place in the 
physical character of the island. But, if we venture to indulge our fancy and regard 
what would have been the inevitable consequences of a gradual upheaval of the land and 
a corresponding extension of its area until it became vastly increased, and its original 
low rounded hills were exalted into mountains, it is plain that a great variety of physical 
conditions would be thereby incurred. One side of the island would be exposed to the 
full force and direct influence of the trade-winds, the other side would be completely 
sheltered from them. The climate of these two portions would accordingly differ, and 
a great difference would be speedily wrought in the character of their .vegetation, while 
that of the lofty central part would undergo a corresponding modification. After some 
longer or shorter period of time, we can conceive the island itself being broken up into two 
portions, separated from one another by a strait, such as divides the North and Middle 
