DARWIN. 69 



In the lowest forms of Metazoa, the body consists 

 throughout life of these two primary germ-layers. But in 

 all higher Intestinal Animals, each of these forms by 

 cleavage two other layers, so that the body is thenceforward 

 composed of four secondary germ-layers. In my " Gastraea 

 Theory " (1873), I have tried to show the general homology 

 of these four layers in all Metazoa, and I have pointed out 

 the important bearing of this fact on the natural system of 

 the animal kingdom. 24 



But though the most important facts in the individual 

 evolution of the human and animal body had been suffi- 

 ciently established by these advances in Animal Ontogeny, 

 yet the most difficult task remained, namely, the discovery 

 of 'the causes by which the evolution of organisms and the 

 production of their forms is effected. The real mechanical 

 causes of individual evolution were first explained in 1859, 

 in Darwin's work, in which the facts of Heredity and 

 Adaptation were for the first time scientifically discussed, 

 and their bearing on Ontogeny correctly interpreted. Only 

 by the Theory of Descent, and by the aid of the laws of 

 Heredity and Adaptation, are we really able to understand 

 the facts of individual evolution, and to explain them by 

 efficient causes. This is the point in which the Darwinian 

 Theory is so important to the History of the Evolution of 

 Man and to the immediate connection of the first part of 

 our science, Germ-history, or Ontogeny, with the second 

 part, Tribal-history, or Phylogeny. 



