DIFFICULTIES OF THE THEORY 149 



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 appa- 

 ratus. 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 man, according to the high authority of Virchow, 

 the beautiful crystalline lens is formed in the embryo by an accu- 

 mulation 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 conclusion regarding the for- 

 mation of the eye, with all its marvellous yet not absolutely per- 

 fect 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 tele- 

 scope. 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 somewhat anal- 

 ogous process. But may not this inference be presumptuous? Have 

 we any right to assume that the Creator works by intellectual 

 powers like those of men ? If we must compare the eye to an optical 

 instrument, we ought in imagination to take a thick layer of trans- 

 parent tissue, with spaces filled with fluid, and with a nerve sensi- 

 tive to light beneath, and then suppose every part of this layer to 

 be continually changing slowly in density, 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 trans- 

 parent layers; and carefully preserving each which, under varied 

 circumstances, in any way or degree, tends to produce a distincter 

 image. We must suppose each new state of the instrument to be 

 multiplied by the million ; each to be preserved until a better one 

 is produced, and then the old ones to be all destroyed. In living 

 bodies, variation will cause the slight alteration, generation will 

 multiply them almost infinitely, and natural selection will pick 

 out with unerring skill each improvement. Let this process go on 

 for millions of years; and during each year on millions of indi- 

 viduals of many kinds ; and may we not believe that a living optical 



