INTRODUCTION. xiii 



As already indicated, the range of variation within this class is extremely 

 limited ; and if our views respecting the taxonomic value of the sub- 

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

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

 of ranking as orders. 



Tlie greatest distinctions existing among the living members of the class 

 are exliibited 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 ; but the differences are so slight, in comparison with those 

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

 diency of giving them that value is extremely doubtful; and tliey can 

 be com1;)ined into one order, wliich may appropriately bear the name of 

 Eurhiijidura. 



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

 divisions of the 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 tlie /«c^ (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 tlie class of birds as now limited confined to the single order of 

 which only we have living representatives. In fossil forms we have, if the 

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

 represented by the genus Archccopteryx and another by the genera Ichthyornis 

 and Apatornis of Marsh. The first has been named Saururcc by Hseckel ; 

 the second Ichthyornithides by Marsh. 



Compelled thus to question the existence of any groups of ordinal value 

 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 land, for wading or for swimming ; or, again, into Land and 

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

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

 such a system as a natural one. 



The time has scarcely arrived to justify 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 tlie class into Land and Water Birds does not express 



