PREFACE. vii 



in a final and positive form by investigation of the succession of 

 life. The record of this succession is contained in the sedimentary 

 deposits of the earth's crust, and is necessarily imperfect. Advance 

 in knowledge in this direction has, however, been very great of re- 

 cent years, so that some parts of the genealogical tree are tolerably 

 or quite complete. We hope reasonably for continued progress in 

 this direction, and if the future is to be judged of by the past, the 

 number of gaps in our knowledge will be greatly lessened. In the 

 absence of the paleontologic record, we necessarily rely on the em- 

 bryologic, which contains a recapitulation of it. The imperfections 

 of the embryonic record are, however, great, and this record differs 

 from the paleontologic in that no future discovery in embryology 

 can correct its irregularities. On the contrary every paleontologic 

 discovery is an addition to positive genealogy. If the present work 

 has any merit, it is derived from the fact that the basis of the argu- 

 ment is the paleontologic record. 



E. D. Cope. 

 Philadelphia, November i, 1895. 



