believing the apparent graduation to be real, when we consider how few comparatively are the still living forms in which the changes cited have been observed, and how far more numerous the extinct forms by which intermediate changes may have been presented. If there be no extravagance in supposing that natural selection may have occasioned these early steps, neither is there any in supposing that, by continued progress in the same direction, it may at length have fabricated the most perfect optical instrument possessed by any member of the _articulata_. And, if credited so far, why not still further? why not with competence to form a man's or an eagle's eye? So far I am as completely at one with Mr. Darwin in respect to the eye as in respect 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