BIOLOGY 



of the theory. We may still hold that, speaking 

 generally, the epidermis and nervous system, 

 together with the most essential parts of the organs 

 of special sense, arise from the outer layer or epi- 

 blast of the embryo ; the lining epithelium of the 

 alimentary canal and its outgrowths from the inner 

 layer or hypoblast ; the skeletal, connective-tissue, 

 muscular, vascular, and excretory systems from the 

 middle layer or mesoblast ; and that these three 

 layers are homologous or morphologically equi- 

 valent throughout the greater part of the animal 

 kingdom. In short, a knowledge of Embryology 

 must still be regarded as an indispensable part of 

 the equipment of the student of Comparative 

 Anatomy and of Organic Evolution. 



If this be true of Embryology, still more so 

 is it of Palaeontology, which brings us into close 

 touch with the actual record of the past history 

 of the organic world as laid down and preserved 

 in the stratified rocks which make up so large a 

 portion of the crust of the earth. The investi- 

 gations of geologists and palaeontologists have 

 accomplished marvellous results in the last few 

 decades. We have long known that the geological 

 record as a whole is entirely in accordance with 

 the theory of organic evolution. Geologists and 

 physicists are now prepared to grant us ample 

 time for the long and tedious processes which 

 the evolution theory postulates. It is, of course, 



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