662 LAND MAMMALS IN THE WESTERN HEMISPHERE 



are the fossil shells of MoUusca, the development of which 

 may often be traced through a thick series of strata, each step 

 of modification being represented by innumerable individuals. 

 In very many instances it appears that each species in a series of 

 successive modifications had many contemporary fluctuating 

 variations, but the change from one species to the next succeed- 

 ing one was by a small though abrupt mutation. The differ- 

 ence between two successive species may be no greater than 

 that between two contemporary variants of the same species, 

 but it was a constant and not a fluctuating difference. There is 

 much reason to believe that such is at least a frequent mode of 

 development, namely, that from species to species and genus 

 to genus the transition has been by slight and sudden changes. 

 The possibility that such abrupt changes, however slight, are 

 illusory and due to small gaps in the record, must be admitted, 

 and though this does not seem to be a very likely explanation, 

 it is given plausibility by the almost perfect continuity between 

 successive species which may sometimes be observed. 



The extremely important and significant distinction be- 

 tween contemporary, fluctuating variations and successive, 

 constant mutations was first drawn by Waagen, who says of 

 them: "One must therefore distinguish strictly between 

 varieties in space and those in time. To describe the former, 

 the long-used name ' variety ' will suffice, for the latter, on the 

 other hand, I would propose, for the sake of brevity, a new 

 term, 'mutation.' A species as such, with reference to its con- 

 nection with earlier or later forms, may be conceived and re- 

 garded as a mutation. But also in regard to the value of these 

 two concepts, just established (variety and mutation), an en- 

 tirely different value is displayed on closer consideration. While 

 the former appears extremely vacillating, of small systematic 

 value, the latter, even though in minute characteristics, is ex- 

 tremely constant and always to be recognized with certainty." ^ 



' W. Waagen, Die Formenreihe des Ammonites subradiatus, Benecke's 

 Geognost.-PalcBont. Beitr., Bd. I, pp. 185-186. 



