Vlii PREFACE 



tire responsibility for the applications of Child's theory brought 

 out in this volume. 



The axial gradient conceptions have appeared to the writer to be 

 strictly in accord with the principles of racial senescence as presented 

 by H. F. Osborn and R. S. Lull in their recent volumes on evolu- 

 tion. The present volume has much to say about adaptive radiation 

 and the various degenerative and senescent conditions so common 

 among vertebrates. The writer has freely made use of the material 

 found in Osborn's and in Lull's books. Many figures have been bor- 

 rowed from both authors, for which the writer is deeply indebted. 



Much of the data used has been taken from the several volumes on 

 vertebrates of the Cambridge Natural History, and from the various 

 comparative anatomies such as those of Wiedersheim, of Kingsley, 

 and of Wilder. Kellicott's " Chordate Embryology " has been found 

 excellent for much of the embryological data. A list of about a hun- 

 dred references to which the writer has had access is presented in an 

 appendix. 



The illustrations have been borrowed to a considerable extent from 

 Macmillan Company publications, but the great majority of the fig- 

 ures have been redrawn by Mr. Kenji Toda to whom I herewith ex- 

 press my hearty thanks. Many of the figures are made up of several 

 related illustrations arranged in such fashion that they readily may 

 be put into chart form. In our own laboratory we are already using 

 a considerable proportion of these compound figures as charts. The 

 sources of these redrawn figures are various and the author wishes to 

 acknowledge with thanks the courtesy of those who have permitted 

 their originals to be thus modified and used. The greatest care has 

 been taken properly to acknowledge the source of each borrowed il- 

 lustration. If in any case the figure has been incorrectly attributed 

 to an author, information regarding the error will be gratefully 

 received. 



The author is not unaware of the shortcomings of the present book. 

 Some critics will doubtless feel that much valuable, and to their minds 

 necessary, data has been omitted. Others will probably believe that 

 much that has been included, especially in connection with the notes on 

 the natural history of certain types, might better have been omitted. 

 But the selection of what to present and what to omit has been care- 

 fully canvassed and what appears to be a workable compromise has 

 been decided upon. The teacher may readily omit what he feels to be 



