Vlil PREFACE. 



the group just mentioned, or he might even light upon the 

 object of his quest at the bottom of the Vertebrate sub- 

 kingdom. 



That these and other similar changes have not been adopted 

 here demands a few words of explanation. In the first place, 

 the present work is intended principally for the guidance of 

 general students, and the author is of opinion that it would 

 for this reason be improper to introduce into it any schemes 

 of classification which have not been accepted with tolerable 

 unanimity by naturalists in general. Most or all of the above- 

 mentioned innovations, however, though supported by many 

 and distinguished names, are opposed by others of equal 

 eminence. They may ultimately turn out to be based on 

 nature, but, in the meanwhile, they have not received anything 

 like universal acceptance. 



In the second place, most of these proposed changes of 

 classification are founded upon a study of the developmental 

 phenomena of animals. Some highly distinguished zoologists 

 hold that embryological characters will ultimately prove to be 

 the true basis of classification ; but in this view the author 

 unfortunately is at present hardly prepared to concur. On the 

 contrary, the author finds himself in the position of being un- 

 able to believe that any general system of classification can 

 maintain its ground unless it be based upon the morphological 

 characters exhibited by adult animals. He would not be held 

 as denying, or even as depreciating, the importance of embry- 

 ological studies, but he is unable to believe that the transitory 

 characters of the young animal can have the same general 

 value in classification for the purposes, at any rate, of ordi- 

 nary students as have the characters drawn from the fully 

 developed organism. 



In the third place, if the author had here adopted one of 

 the most modern classifications of the animal kingdom, as 

 advocated by those who hold that embryology is the true key 

 to taxonomy, he would have no guarantee that he might not 

 be called upon to fundamentally alter his arrangement within 

 a year or two. For embryologists are not agreed as to the true 

 import of the phenomena of the development of many ani- 

 mals, and some of our highest authorities in this department 



