196 THE DOCTRINE OF DESCENT. 



stand the test of the doctrine of Descent and submit to 

 the great general principle. 



If we scrutinize the countless facts of reproduction and 

 development, they certainly admit of classification ; they 

 range themselves in analogous and homologous groups ; 

 types of development become apparent ; we speak of 

 development without metamorphosis, of transformation, 

 and heterogenesis. But what necessary relation the 

 alternating forms, the shapes appearing in heterogenesis, 

 bear to the complete animal or the sexually developed 

 chief representative of the species ? why so many animals 

 undergo no transformations, but emerge " complete; " from 

 the egg ? why the species belonging to the same class 

 or " type " possess the same type of development and 

 process of construction ? — these and similar questions as 

 to the interpretation of the tangled mass of facts press 

 themselves upon us. And they are also tests of our 

 theory of derivation. The doctrine does as much as has 

 been done by any great hypothesis in its special applica- 

 tion ; and if it gives a satisfactory reply to all, or at 

 least to nearly all, pertinent questions, these are so many 

 witnesses and proofs of its truth, which, according to all 

 scientific custom and justice and philosophic method, 

 will remain valid until the falsity of the inductions and 

 inferences has been demonstrated and a better hypothesis 

 substituted in its stead. 



The first proposition derived from the doctrine of 

 Descent in explanation of the facts of individual de- 

 velopment may run thus : accordance in the outlines 

 of development is based on similar derivation; or, 

 somewhat differently stated : accordance in the out- 

 lines of individual development is accounted for by 



