XI PALEONTOLOGY AND EVOLUTION 353 



Thus far . we have been concerned with the 

 intercalary types which occupy the intervals 

 between Families or Orders of the same class; 

 but the investigations which have been carried 

 on by Professor Gegenbaur, Professor Cope, and 

 myself into the structure and relations of the 

 extinct reptilian forms of the Ornithoscelida (or 

 Dinosauria and Compsognatha) have brought to 

 light the existence of intercalary forms between 

 what have hitherto been always regarded as very 

 distinct classes of the vertebrate sub-kingdom, 

 namely Reptilia and Aves. Whatever inferences 

 may, or may not, be drawn from the fact, it is 

 now an established truth that, in many of these 

 Ornithoscelida, the hind limbs and the pelvis are 

 much more similar to those of Birds than they 

 are to those of Reptiles, and that these Bird- 

 reptiles, or Reptile-birds, were more or less com- 

 pletely bipedal. 



When I addressed you in 1862, I should have 

 been bold indeed had I suggested that palaeon- 

 tology 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 doubtful 

 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 



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