DEFINITION OF BIRDS 95 



this Vertebrate series, Birds constitute what is called a highly 

 specialised group ; that is to say, a very particular oifshoot, or, more 

 literally, a side-issue, of the Vertebrate genealogical tree, which in 

 the present geological era has become developed into very numerous 

 (about 10,000) species, closely agreeing with one another in the 

 peculiar sum of their physical character. In comparison with other 

 classes of Vertebrates, all birds are much alike ; there is a less degree 

 of. difference among them than that found among the members of 

 any of the other classes of Vertebrates ; their likeness to each other 

 being strong, and their kind of difference from any other Verte- 

 brates being peculiar, makes them the highly specialised class 

 they are recognised to be. The structural difference between a 

 humming-bird and an ostrich, for example, is not greater in degree 

 than that subsisting between the members of some of the orders of 

 Eeptiles ; whence some hold, with reason, that Birds should not form 

 a class Aves, but an order, or at most a sub-class, of Sauropsida, and 

 so be compared not with a class Eeptilia collectively, but with other 

 sauropsidan orders, such as Ghelonia (turtles), Sauria (lizards), 

 Ophidia (serpents), etc. The practical convenience of starting with 

 a "class" Aves, however, is so great, that such classificatory value 

 will probably long continue to be ascribed, as heretofore, to Birds 

 collectively. I have spoken of Birds as a particular side-issue or 

 lateral branch of the Vertebrate " tree of life " ; hence it is not to be 

 supposed that they are in the direct line of genealogical descent. 

 Though they stand as a group next below Mammals in the scale of 

 evolution, it does not follow that Mammals were developed from 

 any such creature as a Bird has come to be, any more than that 

 Birds have been evolved from any such Eeptiles as those of the pre- 

 sent day. It is one of the popular misunderstandings of the Theory 

 of Evolution, to imagine that all the lower forms of animals are in 

 the genetic line of development of the higher forms ; that man, for 

 example, was once a gorilla or a chimpanzee — actually such an ape. 

 The theory simply requires all forms of life to be developed from 

 some antecedent form, presumably, and in most cases certainly, lower 

 in the scale of organisation. Thus man and the gorilla are both 

 descendants of some common progenitor, more or less unlike either 

 of these existing creatures. All mammals are similarly the modified 

 descendants of some more primitive stock, from which stock sprang 

 also all Sauropsida, mediately or immediately ; therefore a Mammal 

 is not a modified Bird, though higher in the scale ; and, though a 

 Bird is a modified Eeptile, it is not a modification of any such 

 snake or lizard as now exists. The most bird-like reptiles known 

 are not the Pterodactyls, or Flying Eeptiles (Pterosauria), as might 

 be supposed ; but belong to that remarkable order, the Ornitho- 

 scelida, comprising the Dinosaurians, which " present a large series of 



