PREFACE. IX 



of investigation deduce diametrically opposite conclusions 

 from their study of the same phenomena. 



Lastly, there are cases in which the author has preferred to 

 retain even an antique classification, rather than accept any 

 one of many arrangements which are based upon methods of 

 inquiry, which are of the greatest possible value to the com- 

 parative anatomist pure and simple, but which are unavailable 

 for the purposes of those who merely wish to acquire a general 

 but systematic knowledge of Zoology. The class of Birds 

 offers a case in point. In this instance, the author has pre- 

 ferred to retain, with some modifications, an old and only par- 

 tially natural classification, because the only available substi- 

 tutes are arrangements which are purely morphological, and 

 which are based upon the observed variations in single struc- 

 tures. Classifications of this kind, though of the utmost use 

 to the genuine comparative anatomist, can never be thoroughly 

 natural, and they are, at any rate, un suited for any but very 

 advanced students of the science. 



In conclusion, the author can only express his regret that 

 the fact that almost the whole of this work had passed through 

 the press before the middle of October last, should have pre- 

 cluded him from in any way availing himself of, or in s&me 

 cases even from mentioning in the bibliographical lists of refer- 

 ences, some highly valuable works of recent appearance, among 

 which the ' Atlantic/ by Sir Wyville Thomson, the * Morphol- 

 ogy of the Skull,' by Professor Parker and Mr Bettany, and 

 Professor Huxley's 'Comparative Anatomy of Invertebrated 

 Animals,' may be specially alluded to. 



UNITED COLLEGE, ST ANDREWS, 

 Aprils. 1878. 



