HISTORY OF ZOOLOGY. 37 



so must species by continued divergence become so far 

 removed from one another that we distinguish them as 

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

 ferences will become still greater, and cause the establish- 

 ment 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 out 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 

 formtion of several new species through the variability 

 of one species, there must then have been necessary for 

 rendering possible this historical development of the animal 

 and vegetable kingdoms a space of time greater than our 

 mental capacity can grasp, just as the astronomers deal 

 with distances of which we can form no conception. 

 Since now for the idea of the individual development of 

 an animal the special term " Ontogeny" has been chosen, 

 it has also proved convenient to apply to the historical 

 development of animals though this has not been ob- 

 served, but only inferred the special term "History of 

 the Race" or " Phylogcii}'." 1 



Spontaneous Generation. If we attempt to derive all 

 living animals from a common primitive form, we are com- 

 pelled to assume that this was of extremely simple organiza- 

 tion, that it was unicellular; for the simpler the organiza- 

 tion, the less specialized and determined, so much the greater 

 is its capacity for variation. Only from simply-formed 

 organisms can the lower unicellular organisms, the Pro- 

 tozoa, be derived. Finally, for the simply-formed organ- 

 isms alone can we conceive a first natural origin. Since 

 there was undoubtedly a time when upon our earth tem- 

 peratures prevailed which made any life impossible, life 

 upon it must at some time have arisen anew, either through 

 an act of creation or in the natural way through spontane- 

 ous generation. If, in agreement with the spirit of natural 



