w. b. DWToirr. 129-(99) 



gradation, modern science lias turned a very searching 

 light upon the facts bearing on this question. It has had 

 very little respect for any assignments of gradation not 

 founded upon exhaustive research into structure. It has 

 demolished much that .has heretofore been accepted on 

 this subject. It has established new views and princi- 

 ples, which are far more likely to stand the test of time 

 than those which have been superseded. 



It has done this mainly by adopting as the only valid 

 method of reaching the solutions of problems in grada- 

 tion, one that had been only partially and exceptionally 

 applied before. The older philosophers were too much 

 accustomed to compare and classify animals by the re- 

 semblances and differences of external form and attri- 

 butes, and to neglect the study of internal organs and of 

 embryology. It is true that as early as the fourth century 

 before Christ, Aristotle wrote a treatise, "The Natural 

 History of Animals," in which questions of structure 

 and comparative internal anatomy were handled. In 

 this treatise the great philosopher shows a remarkably 

 clear and exact knowledge of certain forms of internal 

 structure as far as that knowledge at that time extended ; 

 yet it was exceedingly meagre as compared with the 

 structural revelations of these later days. 



In the mediaeval ages, no ground was gained. That so 

 obvious a fact as the circulation of the blood was not es- 

 tablished, until Harvey demonstrated it early in the sev- 

 enteenth century, proves the superficial character of 

 biological studies previous to that date. Even after 

 Ray, Leuwenhoek and others, in the latter part of the 

 seventeenth century, had contributed novel and valua- 

 ble researches in this direction, the celebrated scientist 

 Boerhave, early in the eighteenth century, eminent 

 chiefly as a botanist, is said, by publicly belitling the 

 study of comparative anatomy, to have paralyzed its 

 growth for fifty years. 



But this study was revived by Linaeus and Lamarck, and 

 finally established, never to be again checked, early in the 

 ninteenth century, by the great Cuvier. It has been 

 unfolded into majestic proportions by Agassiz and other 

 eminent biologists of our own day. It is now the only 

 accepted method of biological science. It regards ex- 

 ternals as generally of secondary value in classification, 



