I1I8T0RT OF ZOOLOGY. 31 



Phylogeny. — It is now clear that what has here been worked 

 oat in the case of the species must also apply to the other cate- 

 gories of the system. Just as by divergent development varieties 

 become species, so must species by cojitinued divergence become 

 so far removed from one another that wo distinguish them as 

 genera. It will be only a question of time when these differences 

 will become still greater, and cause the establishment of orders, 

 classes, and branches, just as the tender shoots of the young 

 plantlet become in the strong tree the chief branches from which 

 spring lateral branches and twigs. If we pursue this train of 

 thought to its ultimate consequences, we reach the conception that 

 all the animals and plants living at present have arisen by means 

 of variation from a few primitive organisms. Inasmuch as at least 

 many thousands of years are required for the formation of several 

 new species througli the variability of one, there must then have 

 been necessary tor this historical development of the animal and 

 vegetable kingdoms a space of time greater than our mental 

 capacity can grasp. Since for the idea of the individual develop- 

 ment (embryology) of an animal the term Ontogeny has l^een 

 chosen, it has also proved convenient to apply to the historical 

 development of animals — though this has not been observed, but 

 only inferred — the term History of the Eace or Phylogeny. 



Spontaneous Generation. — If we attempt to derive all living 

 animals from a common primitive form, we are compelled to 

 assume that this was of extremely simple organization, that it was 

 unicellular; for the simpler, the less specialized, the organization, 

 so much the greater is its caj^acity for variation. Only from 

 simple organisms can the lower unicellular organisms, the 

 Protozoa, be derived. Finally, for the simple organisms alone 

 can we conceive a natural origin. Siiice there was undoubtedly a 

 time upon our earth when temperatures prevailed which made life 

 imjiossible, life must at some time have arisen, either through an 

 act of creation or in a natural way through spontaneous generation. 

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

 the explanation of natural facts only the forces of nature, we are 

 driven to the hypothesis of spontaneous generation, namely, that 

 by a peculiar combination of materials without life, the compli- 

 cated mechanism which we call ' life ' has arisen. This hypothesis 

 also supposes that the first organisms possessed the simplest con- 

 ceivable structure. 



Variability not proven to be a Universal Principle. — Starting 

 from a basis of facts, by generalization Ave reach a simple concep- 



