96o PRINCIPLES OF STRATIGRAPHY 



influenced by the old inherited ideas of the fixity of specific limits. 

 Palaeontologists are, as a rule, no freer from the shackles of in- 

 herited ideas than are the workers in the morphology and taxonomy 

 of living plants and anim.als. This may in large measure be ac- 

 counted for by the fact that the very recognition of such a thing 

 as a species carries with it the impression of an entity, and the 

 recognition of certain characters as belonging to a species, in a 

 measure carries with it the conception that, if those characters are 

 modified or supplanted by others, the organism in question no 

 longer belongs to that species. 



That the Linn?ean species is a fragment or group of fragments 

 of one or more evolutional series separated from other fragments, 

 in space or time, by the extermination of the connecting links, is 

 pretty generally recognized by naturalists of a philosophical turn of 

 mind. Among such the belief in the nonexistence of species is, 

 theoretically at least, widely held. In other words, naturalists have 

 come to the conclusion that what we call species are merely "snap- 

 shots at the procession of nature as it passes along before us. and 

 that the views we get represent but a temporary phase, and in a 

 short time will no longer be a faithful picture of what really lies 

 before us." "For the procession is moving constantly onward." 

 (Farlow-io.) 



The Mutation of Waagen. 



Waagen in 1868 (44) recognized two kinds of variation within 

 the species — geographic and chronologic. To the former, which 

 comprises the variable members appearing together in the same 

 time period, though they may be geographically separated, he re- 

 stricted the term variation or variety, while for those occurring in 

 chronological succession he proposed the term "mutation." A mu- 

 tation may then be defined as a slightly modified form of the species 

 appearing in a later time-period, and in this sense it has been 

 commonly used by palaeontologists. As an example of a number of 

 mutations appearing in successively higher horizons, the Tertiary 

 series of Paludinas (Vivipara), already referred to, may be cited. 



Palceontologists, whose business it is to study large series of 

 forms from each successive horizon, have since recognized that 

 what Waagen called varieties, in the belief that they had no very 

 definite relationship to each other, are really secondary mutations 

 or sub-mutations (Grabau-17). Thus each developing series has, 

 on reaching a higher horizon, become modified in a certain definite 



