6(3 TEE FACTORS OF ORGANIC EVOLUTION. 



interior solid food, water, and air; while out of the 

 mesoblast are developed structures which have never had, 

 from first to last, any intercourse with the environment. 

 Let us contemplate these general facts. 



Who would have imagined that the nervous system is a 

 modified portion of the primitive epidermis ? In the absence 

 of proofs furnished by the concurrent testimony of embryo- 

 logists during the last thirty or forty years, who would 

 have believed that the brain arises from an infolded 

 tract of the outer skin, which, sinking down beneath the 

 surface, becomes imbedded in other tissues and eventually 

 surrounded by a bony case ? Yet the human nervous 

 system in common with the nervous systems of lower 

 animals is thus originated. In the words of Mr. Balfour, 

 early embryological changes imply that 



&quot; the functions of the central nervous system, which were originally taken 

 by the whole skin, became gradually concentrated in a special part of the 

 skin which was step by step removed from the surface, and has finally 

 become in the higher types a well-defined organ imbedded in the subdermal 

 tissues. . . . The embryological evidence shows that the ganglion-cells of 

 the central part of the nervous system are originally derived from the simple 

 undiJerentiated epithelial cells of the surface of the body.&quot;* 

 Less startling perhaps, though still startling enough, is the 

 fact that the eye is evolved out of a portion of the skin; 

 and that while the crystalline lens and its surroundings 

 thus originate, the &quot; percipient portions of the organs 

 of special sense, especially of optic organs, are often 

 formed from the same part of the primitive epidermis&quot; 

 which forms the central nervous system, t Similarly is it 

 with the organs for smelling and hearing. These, too, 

 begin as sacs formed by infoldings of the epidermis; and 

 while their parts are developing they are joined fromt 

 within by nervous structures which were themselves epi 

 dermic in origin. How are we to interpret these strange 

 transformations ? Observing, as we pass, how absurd from 

 the point of view of the special-creationist, would appear 



* Balfour, I.e. Vol. ii, 400-1. f Balfour, I.e. Vol. ii, p. 401. 



