TRANSFORMISM 233 



definite directions. There are two dominant groups 

 of fishes, the Teleosts and the Elasmobranchs, and 

 both must have originated from a common stock. All 

 the characters in each kind of fish must have been 

 useful (since they were selected) , and all must have been 

 modifications of the characters of the common stock. 

 The latter became modified along two main lines, or 

 directions, which are indicated by the characters of 

 the existing Teleosts and Elasmobranchs. The whole 

 skeleton, the gills, the circulatory system, and the brain 

 differ in certain respects in these groups. Therefore a 

 modification of the brain in the primitive Elasmo- 

 branchs was associated with a modification of the 

 cranium, and therefore with the jaw-apparatus, and 

 so with the branchial skeleton and the gills, and there- 

 fore also with the heart, and so on. Suppose that 

 the evolutionary process included ten useful and co- 

 ordinated variations — not an unlikely hypothesis — and 

 suppose that each of these ten useful variations was 

 associated with nineteen useless ones. The chance 

 that any one of them did occur was therefore one in 

 twenty ; and if they all occurred independently, that 

 is, if the occurrence of any one of them was compatible 

 with the occurrence of any other one. or of all the others, 

 then the chance that all the ten variations occurred 

 simultaneously was 20"'°, that is, one in the number 

 20 followed by 10 cyphers, a rather great improbability. 

 Most biological students are familiar with the 

 similarity of the so-called eye of the mollusc Pecten 

 and that of the vertebrate. The resemblance is one of 

 general structure : in each of these organs there is a 

 camera obscura, a transparent cornea, and behind that 

 a crystalline lens. On the posterior wall of the camera 

 there is a receptor organ, or retina, and this is composed 

 of several layers of nervous elements. The actual 



