ORGANS OF EXTREME PERFECTION. 163 



provement. So the contraction of the iris and the muscular 

 movements of the eye are neither of them essential to vision, 

 but only improvements which might have been added and 

 perfected at any stage of the construction of the instrument.*' 

 Within the highest division of the animal kingdom, namely, 

 the Vertebrata, we can start from an eye so simple, that it 

 consists, as in the lancelet, of a little sack of transparent 

 skin, furnished with a nerve and lined with pigment, but 

 destitute of any other apparatus. In fishes and reptiles, as 

 Owen has remarked, the range of graduation of dioptric 

 structures is very great." It is a significant fact that even 

 in caan, according to the high authority of Virchow, the 

 beautiful crystalline lens is formed in the embryo by an 

 accumulation of epidermic cells, lying in a sack-like fold of 

 the skin ; and the vitreous body is formed from embryonic 

 subcutaneous tissue. To arrive, however, at a just conclu- 

 sion regarding the formation of the eye, with all its marvel- 

 lous yet not absolutely perfect characters, it is indispensable 

 that the reason should conquer the imagination ; but I have 

 felt the difficulty far too keenly to be surprised at others 

 hesitating to extend the principle of natural selection to so 

 startling a length. 



It is scarcely possible to avoid comparing the eye with 

 a telescope. We know that this instrument has been per- 

 fected by the long-continued efforts of the highest human 

 intellects ; and we naturally infer that the eye has been 

 formed by a somewhat analogous process. But may not 

 this inference be presumptuous ? Have we any right to 

 assume that the Creator works by intellectual powers like 

 those of man ? If we must compare the eye t j an optical 

 instrument, we ought in imagination to take a thick layer 

 of transparent tissue, with spaces filled with fluid, and with 

 a nerve sensitive to light beneath, and then suppose every 

 part of this layer to be continually changing slowly in den- 

 sity, so as to separate into layers of different densities and 

 thicknesses, placed at different distances from each other, 

 and with the surfaces of each layer slowly changing in 

 form. Further we must suppose that there is a power, 

 represented by natural selection or the survival of the fit-; 

 test, always intently watching each slight alteration in the. 

 transparent layers ; and carefully preserving each which,-, 

 under varied -circumstances, in any way or degree, tends to 

 rjroduce a distincter image. We must suppose each new 

 state of the instrument to be multiplied by the million^ 



