24 GENERAL PRINCIPLES OF ZOOLOGY 



Since there was undoubtedly a time upon our earth when temperatures 

 prevailed which made life impossible, life must have arisen at some time, 

 either through an act of creation or through spontaneous generation. If, 

 in agreement with the spirit of natural science, we invoke for the explana- 

 tion of natural facts only the forces of nature, we are driven to the hypothe- 

 sis of spontaneous generation, namely, that by a peculiar combination 

 of materials without life, the complicated mechanism which we call 'life' 

 has arisen. 



Starting from a basis of facts, by generalization we reach a simple 

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

 measure departed from the results of direct observation. Observations 

 only show that species are capable of modifications. That this capacity 

 for variation is a principle which explains to us the origin of the animal 

 world, needs further demonstration. 



Proofs of Phylogeny. The evolution of the existing animal world 

 has taken place in the thousands of years long past, but is no longer acces- 

 sible for direct observation, and therefore it can never be followed in the 

 sense that we follow the individual development of an organism. In 

 regard to the conception of the evolution of animals we can merely prove 

 the probability; yet all our observations of facts not only agree with this 

 conception, but find in it their only simple explanation. Such facts are 

 furnished to us by the classification of animals, paleontology, geographical 

 distribution, comparative anatomy, and comparative embryology. 



(1) Proofs from Classification. It has long been recognized, 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 we must have a treelike arrangement, in which the 

 principal divisions, more closely or distantly 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 necessarily 

 leads. 



(2) Paleontological Demonstration approaches nearest to direct proof; 

 for paleontology gives the only traces of existence which the predecessors 

 of the present animal world have left. Even here a hypothetical element 

 creeps in. We can only observe that various grades of forms of an animal 

 group are found in successive strata; if we unite these into a develop- 

 mental series, and regard the younger as derived from the older by varia- 

 tion, we connect the single observations by a very probable hypothesis. 

 But the value of paleontological evidence is weakened much more by its 

 extreme incompleteness. In fossils only the hard parts are generally 



