24 GENERAL PRINCIPLES OF ZOOEOGY 



Since there was undouliledlv a time upon our earth wlien temperatures 

 prevailed which made life impossiljle, 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 (he 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 fads, by generalization we reach a simple 

 conception of the origin of the animal kingdom, ])ut we have iir e(|ual 

 measure departed from the results of direct observation. Observations 

 onh- 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 probaliility; yet all our oljservations of facts not only agree with this 

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

 furnished to us I)Y the classification of animals, paleontologv, geographical 

 distribution, comparative anatomy, and comparative embryology. 



(i) 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 suflicicnt, 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 liml)s, while the smaller 

 branches and twigs correspond to the several classes, orders, etc. This 

 is, in fact, the arrangement to which the theory of e\olution necessarily 

 leads. 



(2) Palconlological Demoiislralion approaches nearest to direct proof; 

 for paleontology gives the only traces of existence which the predecessors 

 of the present animal world have left. E'.ven here a hypothetical element 

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

 group are found in successi\-e 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 prol)able hypothesis. 

 But the value of paleontological e\-idence is weakened much more by its 

 extreme incompleteness. In fossils only the hard parts are generally 



