62 
GENERAL ORNITHOLOGY. 
closely agreeing with one another in the peculiar sum of their physical characters. In compar- 
ison 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 Verte- 
brates : their likeness to each other being strong, and their kind of difference from any other 
Vertebrates being peculiar, makes them the ''highly specialized" class they are recognized 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 Reptiles ; whence 
some hold, with reason, that Birds should not form a class AveSj but an order, or at most a sub- 
class, of Saurdpsida, and so be compared not with a class Reptilia collectively, but with other 
Sauropsidan orders, such as Chelonia (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 Ufe " ? 
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 Reptiles as those of the present day. It is one 
of the popular misunderstandings of the Theory of Evolution, to imagine that all the lower 
fonns 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 or- 
ganization. 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 
;5ome more primitive stock, from which 
stock sprang also all Saui'opsida, medi- 
ately or immediately; therefore, a Mam- 
mal is not a modified Bird, though higher 
in the scale ; and, though a Bird is a 
modified Reptile, it is not a modification 
of any such snake or lizard as now ex- 
ists. The most bird -like reptiles known 
are not the Pterodactyls, or Flying Rep- 
tiles {Pterosaur id), as might be sup- 
posed; but of that remarkable order, the 
Ornithoscelida, comprising the Dinosau- 
rians, which "present a large series of 
modifications intermediate in structure 
between existing Reptilia and Aves," 
and are therefore infereutially in the 
direct ancestral line of modern Birds. 
Fig. 14. —Oldest known ornithological treatise, illus- 
trating also the art of lithography in the Jurassic period, 
engraved by ArcJiceopterijx litJwgraphica. From the original 
slab in the British Museum ; after A. Newton, Ency. Brit. 
Geologic Succession of Birds. — 
Birds have been traced back in geologic 
time to Cretaceous and Jurassic epochs 
of the Mesozoic or Mid- Life period of 
the world's historv- The earliest ornith- 
