136 ORGANS OF EXT11EME PERFECTION. [CHAP. VI. 



although in this case he does not know the transitional states. 

 It has been objected that in order to modify the eye and still 

 preserve it as a perfect instrument, many changes would have 

 to be effected simultaneously, which, it is assumed, could not be 

 done through natural selection ; but as I have attempted to show 

 in my work on the variation of domestic animals, it is not 

 necessary to suppose that the modifications were all simultaneous, 

 if they were extremely slight, and gradual. Different kinds of 

 modification would, also, serve for the same general purpose : as 

 Mr. Wallace has remarked, "if a lens has too short or too long 

 a focus, it may be amended either by an alteration of curvature, 

 or an alteration of density; if the curvature be irregular, and 

 the rays do not converge to a point, then any increased regularity 

 of curvature Avill be an improvement. 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 gradations of dioptric structures is very great." 

 It is a significant fact that even in man, 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 sub-cutaneous tissue. To arrive, however, at a just 

 conclusion 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 perfected by 

 the long-continued efforts of the highest human intellects; and 

 we naturally infer that the eye has been formed by a some- 

 what analogous process. But may not this inference be pre- 

 sumptuous? Have we any right to assume that the Creator 

 works by intellectual powers like those of man ? If we must 

 compare the eye to an optical instrument, we ought in imagin- 

 ation 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 chang- 

 ing slowly in density, so as to separate into layers of different 

 densities and thicknesses, placed at different distances from 





