Birds. 103 



Following up only the Vertebrata branch of our tree, we find 

 the vertebrates conveniently divided into five sub-groups called 

 classes, viz : 



Fishes, Batrachians, Reptiles, Birds and Mammals. 



Now while it simplifies the problem to consider each of these 

 sets of groups as bearing the same relation or value to each other, 

 as ofF-shoots from a common trunk, such, in nature, is not the 

 case, in other words it is difficult to find two groups precisely alike in 

 relative importance, just as it is seldom we find two branches or 

 twigs exactly alike on the real tree. 



The problem then is somewhat more complex than we see 

 it here — and, the method of growth of our tree may be more 

 properly represented somewhat after this fashion (see Fig. 2.) 



To the scientist, birds possess a peculiar interest on account of 

 their apparant isolation from all other classes of Vertebrates; in 

 other words there is an absence, amongst /rc^/// birds at least, of 

 "connecting links," between them and their presumed progenitors, 

 the reptiles — this being indicated on the diagram by the broken 

 lines. 



Amongst recent birds the Penguins and Ostriches approach 

 nearest the reptiles in structure ; but fossil forms are known so 

 intergrading between birds and reptiles, that modern Zoologists 

 have placed all Reptiles and Birds in one "class" called Saitrop- 

 sida. 



In addition to the larger groups into which animals are classi- 

 fied, as shown in our diagram, they are further subdivided (or 

 classified) into numerous smaller ones. Taking the branch (class) 

 "Birds" for instance and tracing it out to its ultimate twigs, 

 we would find it divide into or give off several smaller branches 

 called "Orders," of which there are seventeen represented 

 amongst North American Birds; these seventeen "orders" again 

 give off still smaller divisions called "families" {(id in North 

 America;) the "family" branches give off in their turn "genera" 

 (321 in North America) and each genus sprouts a variable number 

 of "species" (768 in North America, or about 10,000 in the 

 world,) which would represent the terminal twigs of our tree, or 

 that portion of it devoted to the genealogy of birds. 



