238 THE EVOLUTION OF MAN. 



grounds for the assumption that this process is the effect 

 of vitiated Heredity. The simple middle germ-layer of 

 Vertebrates has most probably originated only secondarily 

 by the coalescence of two distinct primary middle layers, 

 and, therefore, the fission of the former into the two latter 

 must be regarded as a tertiary process. 



However this may be, we have now reached the im- 

 portant, definite point in the History of Evolution, in 

 which the whole Vertebrate body, in common with that 

 of most higher animals, forms a tube, the wall of which is 

 composed of four layers, lying one over the other. This 

 is not a figurative comparison; these constituent parts 

 of the tube-wall are actually layers, or thin plates, which 

 lie fixed one over the other. They can even be mechanically 

 parted or split off from each other. This separability is 

 connected with the fact that the cells in each one of the 

 four layers are alike, while those of each are already in 

 some degree distinct or differentiated from those of the 

 other three layers. The first, the skin-sensory layer, con- 

 sists of cells differing from those of the second, or skin- 

 fibrous layer ; the cells of the latter are again different from 

 those of the third, the intestinal-fibrous layer; and these 

 latter are of a somewhat different nature from the cells of 

 the fourth, the intestinal-glandular layer. We find the 

 same four germ-layers as in Man and other Vertebrates 

 (Fig. 51), also in Soft-bodied Animals (Mollusca), Articulates 

 {Arthropuda), Star-animals (Echinoderma), and again in 

 the higher "Worms (Fig. 50). This fact in Comparative 

 Ontogeny is of the greatest pliylogcnetic significance. In 

 all cases, these four secondary germ -layers develop from 

 the two primary germ-layers ; it is only in the lower Plant- 



