PREFACE. 



THE present book is an attempt to select from the mass of 

 facts accumulated by biologists, those which, in the author's 

 opinion, throw a clear light on the problem of organic evolution, 

 and especially that of the animal kingdom. As the actual lines of 

 descent can be finally demonstrated chiefly from paleontologic re- 

 search, I have drawn a large part of my evidence from this source. 

 Of course, the restriction imposed by limited space has compelled 

 the omission of a great many facts which have an important bear- 

 ing on the problem. I have preferred the paleontologic evidence 

 for another reason. Darwin and the writers of his immediate 

 school have drawn most of their evidence from facts which are 

 embraced in the science of oecology. Weismann and writers of 

 his type draw most of their evidence from the facts of embryology. 

 The mass of facts recently brought to light in the field of paleon- 

 tology, especially in the United States, remained to be presented, 

 and the evidence they contain interwoven with that derived from 

 the sources mentioned. 



Many of the zoologists of this country, in common with many 

 of those of other nations, have found reason for believing that the 

 factors of evolution which were first clearly formulated by La- 

 marck, are really such. This view is taken in the following pages, 

 and the book may be regarded as containing a plea on their behalf. 

 In other words, the argument is constructive and not destructive. 

 The attempt is made to show what we know, rather than what we 

 do not know. This is proper at this time, since, in my opinion, a 

 certain amount of evidence has accumulated to demonstrate the 

 doctrine here defended, and which I have defended as a working 

 hypothesis for twenty-five years. 



In the following pages I have cited many authors who have 

 contributed to the result, but it has been impossible to cite all who 



