1220 CLASS AVES. 



the jaw was still variable. There is no reason to think that at that period 

 any Reptile (with the exception of Pterodactyles, which, as has already 

 been said, are certainly not in the line of Birds' ancestors) had a keeled 

 sternum. Hence it seems almost impossible that the first Bird should 

 have had one ; that is to say, it must have been practically of the Ratite 

 type. Professor Marsh has shown that there is good reason for believing 

 that the power of flight was gradually acquired by Birds, and with that 

 power would be associated the development of a keel to the sternum, on 

 which the volant faculty so much depends. . . . Thus the Carinate type 

 would, from all we can see at present, appear to have been evolved from 

 the Ratite." After observing that embryological and distributional facts 

 afford support to this view, Professor Newton continues as follows : 

 " No doubt the difficulty presented by the biconcave vertebrae of the ear- 

 liest known representative of the Carinate type is a considerable obstacle 

 to the view just taken. But Professor Marsh has shown that in the third 

 cervical vertebra of Ichthyornis ' we catch nature in the act as it were ' 

 of modifying one form of vertebra into another, for this single vertebra 

 in Ichthyomis'is in vertical section ' moderately convex, while transversely 

 it is strongly concave, thus presenting a near approach to the saddle-like 

 articulation;' and he proceeds to point out that this specialised feature 

 occurs at the first bend of the neck, and, greatly facilitating motion in a 

 vertical plane, is ' mainly due originally to its predominance.' The form 

 of the vertebrae would accordingly seem to be as much correlated with 

 the mobility of the neck as is the form of the sternum with the faculty of 

 flight. If, therefore, the development of the saddle shape be an indica- 

 tion of development, as well may be the outgrowth of a keel." In conclu- 

 sion, the Professor observes that the question must be regarded as still 

 unsettled, although his own opinion is strongly in favour of the Ratite 

 being the earlier type. 



On the other hand, Dr Gadow, in a communication of later date, con- 

 cludes that the Ratitae were most probably descendants of Birds which 

 formerly possessed the power of flight ; this view being said to be sup- 

 ported by the structure of the wings, and the nature of the feathers of the 

 young. 



In the following chapter a brief summary is given of the chief 

 divisions of Birds, with mention of those families known to be re- 

 presented in a fossil state. It would, however, exceed the limits of 

 this work to give even the leading osteological characters of such 

 families, since, owing to the great general similarity in the structure 

 of all Carinate Birds, such characters could only be indicated by the 

 introduction of a great mass of detail. 



It should also be observed that the majority of writers rank the 

 three primary divisions of Birds as subclasses, and the secondary 

 divisions as orders, with the proviso that such orders are of very 

 different value from those of Reptiles. With the object of avoiding 

 this inequality the view of Professor Huxley, who has termed the 

 primary divisions orders and the secondary ones suborders, has been 

 followed in this work. 



