INTRODUCTION. xiii 



As already indicated, the range of ^'ariation within this class is extremely 

 limited ; and it our views respecting the tuxunomic value of the sub- 

 divisions are infiueuced by this condition of things, we are obliged to deny 

 to the groups of living birds the right which has generally been conceded 

 of ranking as ortlers. 



The greatest tlistinctions existing among the living members of the class 

 are exhibited on the one hand by the Ostriches and Kiwis and the related 

 forms, and on the other by all the remaining birds. 



These contrasted groups have been regarded by Professor Huxley as of 

 ordinal value; l)ut the differences are so slight, in comparison with those 

 which have received ordinal distinction in other classes, that the expe- 

 diency of gi^'ing them that value is extremely doubtful ; and they can 

 be combined into one order, wliich may appropriately bear the name of 

 £i'rhipidi'r/i. 



An objection has been urged to this depreciation of the value of the sub- 

 divisions of tlie class, on the ground that the peculiar adaptation for flight, 

 which is the prominent characteristic of birds, is incapable of being combined 

 with a wider range of form. This is, at most, an explanation of the cause 

 of the slight range of variation, and should not therefore affect the exposi- 

 tion of the fact (thereby admitted) in a classification based on morphological 

 characteristics. But it must also be borne in mind that flight is by no 

 means incompatible with extreme modifications, not only of the organs of 

 flight, but of other parts, as is well exemplified in the case of bats and the 

 extinct pterodactyls. 



Nor is the class of birds as now limited confined to the single order of 

 M'hicli only we liave living representatives. In fossil forms we have, if the 

 differences assumed be confirmed, types of two distinct orders, one being 

 represented by the genus Arclta-optcryj: and another Ijy the genera Ichthi/ornis 

 and Apatornis of Marsh. The first has been named Sanrurw by Hffickel ; 

 the second Irhthyornifhidcs by Marsh. 



Compelled thus to question the existence of any groups of ordinal A'alue 

 among recent birds, we proceed now to examine the grounds upon which natu- 

 ral subdivisions should be based. The prominent features in the classification 

 of the class until recently have been the divisions into groups distinguished 

 by their adaptation for different modes of life ; that is, whether aerial or for 

 progression on laud, for wading or for swimming ; or, again, into Land and 

 Water Birds. Such groups liave a certain value as simjily artificial combi- 

 nations, but we must not be considered as thereby committing ourselves to 

 such a system as a natural one. 



Tlie time has scarcely arrived to ju.stify any system of classification 

 hitherto proposed, and we can only have a sure foundation after an exhaust- 

 ive study of the osteology, as well as the neurology and splanchnology, of the 

 various members. Enough, however, has already been done to convince us 

 that the subdivision of the class into Land and Water Birds does not express 



