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 
