THE APPLICATION OF THIS PRINCIPLE, 147 



postponed formulating any more histories of ani- 

 mals, until such time as the abundance of evidence 

 shall enable them to reach more legitimate conclu- 

 sions. In short, although embryology is believed to 

 be a key to animal history, it proves so difficult to 

 use that many doors still remain locked. 



The Application of this Principle, 



Having recognized the parallel, and having con- 

 sidered its significance, we have stated the whole of 

 the direct argument which embryology has to offer. 

 It is impossible, however, to get a correct idea of the 

 real force of the argument without examining some 

 of the results of the application of the principle. 

 We will now, therefore, proceed to consider what 

 embryology has taught us of the history of animals. 



All embryological discussion is now based on the 

 tacitly understood assumption, that an animal's de- 

 velopment is a repetition of the history of its ances- 

 tors, and that a complete knowledge of the former 

 would give us an approximately perfect knowledge 

 of the latter. By a complete knowledge of develop- 

 ment is not meant simply the knowledge of the 

 present actual course of embryonic growth, for 

 when the possibility of the modification of the 

 record is recognized, it becomes plain that the 

 knowledge of the actual embryology is by no means 

 a complete understanding of its development. A 

 complete knowledge would imply a knowledge of 

 the primitive type of development, and the method 

 by which the modified type has been derived, to- 

 gether with the circumstances accompanying the 



