248 CLAIMS OF OOLOGY TO BE AN EXACT SCIENCE. 
concern the latter as wellas the former. Sorby discovered 
in the shell no fewer than seven constituents found in the body 
of the bird analogous to the composition of blood and bile; and 
Wickmann maintains that colour originates from the ovary 
itself, and that the very source of colouring in birds’ eggs is 
the blood. ‘“ Natural Selection,” says Romanes, “is primarily 
a theory of the cumulative development of adaptations 
wherever these occur, and therefore is only incidentally or 
otherwise a theory of species in cases where allied species 
differ from one another in respect of peculiar characters, 
which are also adapted characters.” May it not be that 
the appearance of the egg exactly defines the predominance 
of those characters? If the eggs of the Song Thrush differ 
conspicuously from those of all other European members of 
the genus, may not such a peculiarity be due to the fact 
that in that particular species the colour which distin- 
guished the original type has by some means been pre- 
served; or, conversely, that in a highly specialised form, 
such characters as determine the retention of colour have been 
gradually lost in the protracted progress of polytypic evolution ? 
Such questions are purely speculative, and there is little 
hope of their being satisfactorily answered until the develop- 
ment theory has advanced far beyond its present rudimen- 
tary state. But whether, as Weismann and Russel Wallace 
maintain, Natural Selection is the sole cause of the modifi- 
cation of species, or whether, as Darwin and Romanes have 
argued, it is only one of many forms of what the latter 
has called “ Discriminate Isolation,” it is at least certain 
that all such changes are controlled by one great and in- 
fallible principle-—the principle which not only sways the 
physical universe, visible to us, or extending far beyond our 
little ken, but which, if understood, might explain the 
action or fix the domain of those stupendous mysteries 
which men of science have often arrogantly despised as 
metaphysical or supernatural, although inseparably and in- 
dissolubly connected with organic matter—the principle of 
the subjection of lower to higher law. 
