QQ JOUKNAL OF THE MiTCHELL SoCIETY [AuQUSt 



compreliensive and detailed, forms an immense part of the 

 safe, secure basis on which rest all the biological inquiries of 

 today. As to the part that American zoologists have played in 

 the development of morphology, while it cannot be claimed that 

 they brought to light any of the very greatest generalizations, 

 comparable with those of von Baer, Rathke, Haeckel, and Kow- 

 alevsky, the world owes to them a large number of acute and 

 important investigations. In addition to the names I have 

 already mentioned in speaking of the progress of embryology, 

 two great Americans, who have recently died, should be referred 

 to here, W. K. Brooks and C. O. Whitman. 



It must not be supposed that the work in descriptive mor- 

 phology is at an end. By no means ! There is so much to be 

 done that it will doubtless occupy many zoologists for centuries. 

 But as was the case earlier with systematic zoology, so later 

 with pure morphology: it no longer occupies the centre of 

 the stage. 



In dealing with evolution I have spoken as if the effort had 

 been solely to reconstruct in the imagination the past world 

 and so to discover the kinship between groups and their place 

 in a natural classification. But along with this inquiry went 

 always the question. What were the agencies that have been at 

 work in the transformation of species ? This is perhaps the 

 question of more practical concern to us, for the agencies that 

 have been at work in the past are doubtless at work today. It 

 has been easier, however, to learn something fairly definite about 

 the course of evolution than it has been to determine the factors 

 at work. Moreover, it is doubtless true that a knowledge of 

 lines of ancestry will aid us in the inquiry into the nature of 

 these factors. It is not difficult then to understand why atten- 

 tion was concentrated so long on the data of comparative 

 morphology. 



Zoologists and paleontologists have nevertheless speculated 

 abundantly on the causes of transformation, making use of such 

 knowledge as has been available. So in this country Cope, 

 Hyatt, and others have developed theories dealing with these 

 problems. The theories of Cope and Hyatt proceed in part 



