142 SHOULDER-GIRDLE AND BREAST-BONE. 



BI11DS. 



GENERAL REMARKS. 



ONCE amongst the Warm-blooded Vertebrata, we encounter a host of morphological difficulties, 

 arising from metamorphic changes very analogous to the modifications undergone by Insects 

 as they pass from their larval and pupal condition into their perfect or " imago " stage. 



There are genera, however, both amongst the Birds and the Mammalia, in which but little 

 of this kind of development takes place ; the arrested, simple conditions of the Reptilia being 

 permanently retained : these generalised types are the typical Struthionidae and the Mono- 

 tremata. 



These generalised types and the Crocodiles have the most remarkable morphological affinities, 

 and throw mutual light upon each other. As for Birds in general, it is hard to say which is the 

 most Reptilian group ; for I have found the most unmistakable Lacertian characters in the 

 noblest aerial types, whilst the Struthionidse, which undergo the least metamorphosis, come as 

 near to the Mammalia as they do to the Reptilia. 



In studying the lower half of the thorax and the Shoulder-girdle of the Bird, I shall " fetch 

 a compass " round the entire class, beginning with the Penguin, and ending with the Ostrich. 

 I do not, however, intend to make merely a " coast survey," but to travel inland also at various 

 points, so as to learn something of the central tribes. 



In so doing, I must refer the reader from time to time to the territories we have left behind, 

 and occasionally to that towards which we are led through the various highways and byways of 

 the Bird-class. We might indeed gain the Mammalian Class by a very short route ; for we have 

 but to step from the Crocodile to the Ostrich, and from the Ostrich to the Monotreme, and we 

 are landed amongst the creatures that " make their teats naked, and give suck to their whelps ;" 

 but this is not the right way, for every finished and noble Bird-type would be left on the right 

 hand and on the left. 



The changes that these higher morphological types have undergone are not brought about 

 by the adoption of new structures, nor by leaving out the old, but by segmentation, arrest, and 

 metamorphosis; the "raw material" is taken up again from those larval and pupal types, the 

 Fishes, Amphibians, and Reptiles ; but the primordial skeletal masses are cloven, selected, and 

 brought into new and closer correlation, so that their original Reptilian and Ichthyic conditions 

 have to be sought for in their early and rapidly changing stages. The skeletal regions which 

 have to be treated of here may be clearly characterised in the Bird-class ; for Birds in spite of 

 their potency in generic and specific numbers, have, on the whole, much uniformity. 



For instance, transverse segmentation of the Shoulder-girdle moiety, which we have lost sight 

 of ever since we left the Fish-class, reappears, and is very constant in the Birds ; for, except in the 

 typical StruthionidaB, the scapula and coracoid are always completely cleft asunder. The next 

 character is universal, and it is this, namely, that the supra-scapula is ossified continuously from the 



