32 GENERAL PRINCIPLES OF ZOOLOGY. 



tion of the origin of the animal kingdom, but we have in equal 

 measure departed from the results of direct observation. Observa- 

 tions only show us that species are capable of changes and can 

 from themselves produce new species. That this capacity for 

 variation is a universal principle, a principle which explains to us 

 the origin of the animal world, needs further demonstration. 



Proofs of Phylogeny. The rise of the existing animal world 

 is a process which has taken place in the thousands of years long 

 past, but is no longer accessible for direct observation, and there- 

 fore it can never be proved in the sense that we explain the indi- 

 vidual development of an organism. In regard to the conception 

 of the simple evolution of animals we can merely prove the 

 probability; yet it is shown that all our observations of accessible 

 facts not only agree with this conception, but find in it their only 

 simple explanation. Such facts are furnished to us by the classi- 

 fication of animals, paleontology, geographical distribution, com- 

 parative anatomy, and comparative embryology. 



(1) Proofs from Classification. For a long time it has been 

 recognized, and in recent times finds ever-increasing confirmation, 

 that if we wish to express graphically the relationships of animals, 

 their classes, orders, genera, and species, simple co-ordination and 

 subordination are not sufficient, but one must select a treelike 

 arrangement, in which the principal divisions, more closely or dis- 

 tantly related to one another, the branches, phyla, or types, 

 represent the main limbs, while the smaller branches and twigs 

 correspond to the several classes, orders, etc. This is, in fact, 

 the arrangement to which the theory of evolution, as seen above, 

 necessarily leads. 



(2) Paleontological Demonstration approaches nearest to what 

 one might call direct proof; for paleontology gives us the only 

 traces of existence which the predecessors of the present animal 

 world have left. Even here a hypothetical element has crept into 

 the demonstration. We can only observe that various grades of 

 forms of an animal group are found in successive strata; if we 

 .unite these into a developmental series, and regard the younger 

 as derived from the older by variation, we depart, strictly speak- 

 ing, from the basis of fact. But the value of paleontological 

 evidence is weakened much more by its extreme incompleteness. 

 In fossils only the hard parts are generally preserved; the soft 

 parts, on the other hand, which alone are present, or at least make 

 up the most important part of many animals, are almost always 

 lost. Only rarely are the soft parts (muscle of fishes, ink-bag of 



