HISTORY OF ZOOLOGY. 39 



types, represent the main branches of the tree, while the 

 finer branches correspond to the several classes, orders, 

 etc. This is, in fact, the arrangement to which the de- 

 scent theory, as seen above, necessarily leads. 



(2) Paleontological Demonstration approaches nearest to 

 what one might call direct proof; for Paleontology gives 

 us the last traces of existence which the predecessors of 

 the present animal world have left. Meanwhile we must 

 take into consideration the fact that even here a hypo- 

 thetical element has crept into the character of the demon- 

 stration. We can only observe that various grades of 

 form of an animal group are found in successive strata; if 

 we unite these grades of form among themselves into a 

 developmental series, and think of the younger as derived 

 from the older by variation, we depart, strictly speaking, 

 from the basis of fact. But much more is the value of 

 paleontological evidence weakened by its extreme incom- 

 pleteness. In fossils only the hard parts of animals are 

 generally preserved ; the soft parts, on the other hand, which 

 in many branches alone are present, or at least make up 

 the most important part of their organization, are almost 

 always lost. Even the hard parts preserve good articula- 

 tions only under exceptionally favorable conditions. If 

 further we take into consideration the fact that these 

 treasures are buried in the bosom of the earth, and are 

 usually obtained only by accident, in quarrying and road- 

 building, and besides only extremely seldom excavated 

 with care and with scientific deliberation, it becomes suffi- 

 ciently clear how little is to be expected from the past and 

 indeed future material of Paleontology. 



Examples of Paleontological Proof. Yet Paleontology 

 has already furnished many important proofs of the Theory 

 of Descent. It has shown that the lower forms appeared 

 first, and the higher organized only later. Among ani- 

 mals in general the latest to appear were the vertebrates, 

 and of these the mammals ; among the mammals the apes 

 and man. For smaller groups it has indeed been fortunate 

 in collecting genealogical material. Transitional forms lead 



