. HISTORY OF ZOOLOGY 23 



species vary, i.e., compared with one another they attain greater or less 

 differences. So long as the extreme differences are Ijridged by transitional 

 forms we speak of varieties of a species; if, on the other hand, the inter- 

 mediate forms have died out, or were not present in the beginning, and the 

 differences have in the course of time become fixed, and so intensified 

 that a sexual union of the extreme forms results either in complete 

 sterility or at least in a marked tendency towards sterility, then we speak 

 of different species. 



Species may be Related to each other in Unequal Degrees. — In 

 favor of this view, that varieties will in longer time become species, is the 

 great agreement which usually exists between the two. In genera which 

 comprise a larger number of species, the species usually show also many 

 varieties; the species are then usually grouped in sub-genera, i.e., they are 

 related to each other in unequal degrees, since they form small groups 

 arranged around certain species. With varieties the case is similar. In 

 such genera the formation of species is in active progress; but each 

 species formation presupposes a high degree of variability. 



Phylogeny. — It is clear that what has here been worked out for the 

 species must also apply to the other categories of the system. Just as by 

 divergent development varieties become species, so must species by con- 

 tinued divergence become so far removed from one another that we dis- 

 tinguish them as genera. It is only a question of time when these diff'er- 

 ences will become still greater, and give rise to orders, classes, and branches, 

 just as the tender shoots of the young plant become in the tree the chief 

 branches from which spring lateral branches and twigs. If we pursue this 

 train of thought 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 thousands of years are required for the forma- 

 tion of new species through the variability of one, there must have been 

 necessary for this historical development of the animal and vegetable 

 kingdoms a space of time greater than our mental capacity can grasp. 

 Since for the individual development (embryology) of an animal the term 

 ontogeny has been chosen, it has also proved convenient to apply to the 

 historical development of animals the term History of the Race or 

 Phylogeny. 



Spontaneous Generation. — If we attempt to derive all living animals 

 from a common ancestor, we must assume that this was extremely simple, 

 that it was unicellular; for the simpler, the less specialized, the organiza- 

 tion, so much the greater is its capacity for modification. Only from 

 simple organisms can the unicellular organisms, the Protozoa, be derived. 

 Finally, for the simple organisms alone can we conceive a natural origin. 



