26 EXPERIMENTAL EVOLUTION LECT. 



other hand, our geological notions may require im- 

 portant alterations as concerns synchronism and 

 heterochronism of the strata and of their contents. 

 It may happen that vestiges of animals which we 

 consider as very recent may be found in much older 

 strata ; it may also happen that some types have 

 been evolved in very limited portions of the globe 

 at different times and with different characters. It 

 may be granted that our geological conceptions re- 

 quire to be revised, and in many cases altered. But 

 however fragmentary and imperfect our present know- 

 ledge may be, it nevertheless yields important con- 

 clusions. Through palaeontology we perceive in some 

 cases the passage from one group of animals to 

 another, and while theory shows that birds are 

 probably in close relationship with reptiles, the 

 Jurassic strata yield ArcJiceopteryx lithographic^ 

 which partakes of the character of both groups, and 

 in the more recent Tertiary deposits we meet with 

 many forms which have now disappeared, but are in 

 intimate connection with the existing species of many 

 orders, and seem positively to be the ancestors from 

 which the latter have been evolved with slight modifi- 

 cations. It is enough to recall the important investi- 

 gations of Gaudry, Leidy, Falconer, Cope, Marsh, 

 Boyd-Dawkins, and Lartet, who have traced, with the 

 utmost probability, the exact line of descent from 



