CLASSIFICATIONS IN \' AT URAL HISTORY. 20/ 



Species of the Paleontologist. — We have already considered 

 the philosoj^hical notion of species, but the real species which 

 we deal witli in raleontology is, as defined by Zittel, all those 

 individuals or all fragments which present certain common 

 characters and form a circumscribed group, independent of 

 geological range or geographical distribution, and which may 

 be linked with allied species by a small number of intermedi- 

 ate forms. If in the same species certain individuals possess 

 some peculiar characters which are more or less conspicuous, 

 they constitute varieties or raees of the naturalists. The va- 

 rieties maintain in some cases the same habitat with the stock 

 form, in other cases they live in different regions {representa- 

 tive varieties). It is more difficult for the paleontologist than 

 for the zoologist to distinguish species from varieties. It 

 often happens that there are in two contiguous formations 

 fossils of the same genus, presenting differences, very slight 

 but constant, in which case they should be distinguished as 

 separate species. Fossil species are not always restricted to 

 -either a single geological horizon or bed, nor are they con- 

 fined to the same geographical region. 



Varieties. — The same fact applies in some measure to vari- 

 eties. Those slight differences, observed upon comparing the 

 representatives of a species coming from different strata or from 

 different regions, are considered to be varietal, and not spe- 

 cific, in case the differences consist in unequal degrees of 

 modification of the same part or parts, so that the several 

 specimens may be arranged in a continuous series connect- 

 ing the extremes by intermediate forms. When such a series 

 of forms of one species exhibits the differences in connection 

 with geographical distribution only, the degrees of modifica- 

 tion are defined as varietal, and those prominent in a particu- 

 lar locality may then be called distinct varieties. 



Mutations. — -When the modification of form is observed to 

 be associated with succession of their appearance, the differ- 

 ences are called imitations — a term proposed by Waagen. 

 Thus modifications of specific form, when contemporaneous, 

 are called varieties or variations; when successive in time 

 they are called mutations. 



The History of Organisms; the Two Methods of ite Study. — The 



