PREFACE ix 



owe for the most part to the artistic skill, combined with high 

 scientific accuracy, of Mr. Kirkpatrick Maxwell. Apart from the 

 completely original figures it will be- noticed that there are many 

 which have been worked up from illustrations in original papers, 

 but which are practically new figures. In all such cases, however, 

 I have thought it only right to make due acknowledgment of the 

 author of the original figure. * 



For permission to borrow particular text-book figures I have to 

 thank Professor Frank E. Lillie, Mr. John Murray, Messrs. Masson 

 & Cie, and Messrs. Macmillan. The present unfortunate circum- 

 stances of international strife call for a special acknowledgment of 

 the generous way in which Professor Alf. Greil entrusted to me 

 the originals of his valuable unpublished figures illustrating the 

 development of the heart in the bird. They are reproduced on 

 pages 384 and 385. 



I have included the name of Charles Darwin in the dedication 

 of this volume to emphasize the fact that Embryology is primarily 

 a branch of synthetic evolutionary science. While the fashion of 

 the day in evolutionary research favours rather experimental 

 research into the phenomena of inheritance and more or less 

 speculative enquiry into the ultimate mechanism of inheritance or 

 into the possible causes of evolutionary change— morphology, and 

 more especially embryology, is steadily at work all the while, mapping 

 out the paths along which the evolution of organisms and their con- 

 stituent organs has taken place. Working away in comparative 

 seclusion, unadvertised, and for the most part unnoticed, embryology 

 is thus building up an important part of the framework of what 

 will be the permanent edifice of evolutionary science. 



J. GRAHAM KEEP. 



February 3, 1919, 



