espect to
any other of the subjects taken by him for illustration. The fact is,
however, that in this, as in every similar instance, he has completely
evaded the real difficulties of the case. It is not a whit more
startling to be told that the most complex eye, with all the latest
improvements, than to be told that the earliest rudiment of an optic
nerve, may have been formed by the gradual accumulation of minute
differences. Only allow time enough for the requisite accumulation, and
neither operation is one whit more unintelligible than the other. The
difficulty, equally and utterly insuperable in both cases, is to
understand how the difference can have been undesigned. 'How a nerve
comes to be sensitive to light,' says Mr. Darwin, 'hardly concerns us
more than how life itself originated.' Perhaps not; nor, indeed, very
well could it, for the second question of the two is surely one of
almost unsurpassed concernment; but, at any rate, when either of the two
is asked, nothing can be more reprehensible than, by studiously ignoring
the only alternative reply, to leave it to be inferred that the nerve
made itself, or that life caused itself to live, that both are in short
examples of what Mr. Darwin strangely calls '_variation causing
alterations_.'[41] Let us briefly consider a few of the results supposed
to be attributable to this singular process. The eye, as every reader of
course knows, though here and there one perhaps may not be the worse for
being reminded, consists of four coats--the _sclerotic_, outermost and
strongest, which constitutes the white of the eye; the circular, tough,
and coloured, yet pellucid, _cornea_, in the centre of which is seen the
pupil; the _choroid_, full charged with black pigment, and lining the
sclerotic; the _retina_, an expansion of the optic nerve, lining in its
turn the choroid; of the _iris_, a flat membrane, dividing the eye into
two very unequally-sized chambers; of a lens termed the _crystalline_,
suspended in the posterior chamber immediately behind the iris; and of
two humours (also virtual lenses), whereof one, the _aqueous_, is
enclosed in the anterior chamber formed by the iris and the cornea,
while the other, the _vitreous_, fills the whole of the posterior
chamber save what is occupied by the crystalline lens. By what nice
interlacement of filaments the fibrous ring that margins the pupil, or
aperture through the iris, regulates the admission of light, contracting
or expan