ix.] PALEONTOLOGY AND EVOLUTION. 191 



been always regarded as very distinct classes of the 

 vertebrate sub-kingdom, namely ReptiUa and Aves. 

 Whatever inferences may, or may not, be drawn from 

 the fact, it is now an established truth that, in many 

 of these OmitkosceKdd, the hind limbs and the pelvis 

 are much more similar to those of Birds than they are 

 to those of Eeptiles, and that these Bird-reptiles, or 

 Kep tile-birds, were more or less completely bipedal. 



When I addressed you in 1862, I should have been 

 bold indeed had I suggested that palaeontology would 

 before long show us the possibility of a direct transition 

 from the type of the lizard to that of the ostrich. At 

 the present moment we have, in the Ornithoscelida, the 

 intercalary type, which proves that transition to be 

 something more than a possibility ; but it is very doubt- 

 ful whether any of the genera of Ornithoscelida with 

 which we are at present acquainted are the actual linear 

 types by which the transition from the lizard to the bird 

 was effected. These, very probably, are still hidden from 

 us in the older formations. 



Let us now endeavour to find some cases of true 

 linear types, or forms which are intermediate between 

 others because they stand in a direct genetic relation to 

 them. It is no easy matter to find clear and unmis- 

 takable evidence of filiation among fossil animals; for, 

 in order that such evidence should be quite satisfactory, 

 it is necessary that we should be acquainted with all 

 the most important features of the organization of the 

 animals which are supposed to be thus related, and not 

 merely with the fragments upon which the genera and 

 species of the palaeontologist are so often based. M. 

 Gaudry has arranged the species of Hywnidce, Probos- 

 cidea, Rhino cerotidce, and Equidce in their order of 

 filiation from their earliest appearance in the Miocene 

 epoch to the present time, and Professor Etitimeyer has 



