VON ZITTEL'S "HANDBUCH" 113 



of descent is alone capable of giving a rational ex- 

 planation of these series of forms. 



In the same way the resemblance of the faunas 

 of the same geological age and the geographical dis- 

 tribution of extinct and living animals bring very 

 strong arguments in favour of this theory. But, 

 after all, we cannot forget that there exists an 

 immense number of creatures without intermediate 

 links, and that the relations of the great divisions of 

 the animal or vegetable kingdom are much less 

 strict than the theory demands. Even the Archae- 

 opteryx, the sensational discovery of which es- 

 tablished a relationship between two such distinct 

 classes as Birds and Reptiles, but very imperfectly 

 bridges over this gap, and does not indicate to us 

 the point of bifurcation of these two classes. Inter- 

 mediate links are wanting between Amphibians and 

 Reptiles. Mammals, likewise, are very isolated, and 

 the wide gap which separates them from the other 

 Vertebrates cannot be contested by any zoologist. 

 We do not even know with certainty a single type 

 of mammal so nearly approaching to the lower 

 Vertebrates as is the present Ornithorhyncus. The 

 keenest partisans of the descent theory must ac- 

 knowledge that the fossil links between the classes 

 and orders of the two kingdoms exist in infini- 

 tesimally small numbers. 



There exists, it is true, within the great groups, 

 series of forms which not only show the plasticity 

 of beings, but also inform us of the processes 

 by which these series have transformed them- 

 selves in the course of time. But, even on these 

 lines, it is easier to accumulate probabilities than 

 l 



