﻿Ixxii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I905, 



plane over a wide area separating fauna from fauna, any more than 

 we can define such a plane separating beds of one lithological type 

 from those of another over wide tracts of country. 



A short space may be occupied with some remarks upon the 

 separation of fossils into species. In the case of or- 

 ganisms of which the hard parts only are preserved, it is difficult 

 to distinguish between species and varieties. For purposes of 

 classification of the sediments, however, it is immaterial whether a 

 particular assemblage of individuals can or cannot be referred 

 to as a species, so long as these individuals present certain 

 characters in common whereby they can be identified, and also dis- 

 tinguished from allied forms. Even if the assemblage constitutes 

 only a variety from another assemblage, so long as it is found 

 in beds of different age from those which contain that other 

 assemblage, it is useful for classifying the beds. Whether these 

 varietal assemblages coexisted with the allied forms, or lived sub- 

 sequently to them, is a matter for determination by observation. 



The study of mutations has already been carried on to some 

 extent, but much more work remains to be done in this direction. 



The mention of mutations reminds me that there has been some 

 misconception concerning the use of the terms mutation and 

 cir cuius. As it is most desirable to have a clear conception 

 as to the proper signification of these terms, I have applied to 

 Dr. F. A. Bather, who has kindly supplied me with definitions of 

 the terms and a discussion thereon. I offer my sincere thanks to 

 him for these notes, which provide geologists with a clear statement 

 of the precise meaning of the terms, drawn up by a recognized 

 authority. Dr. Bather writes : — 



'The term mutation was introduced by W. Waagen in 1869, on p. 186 

 •of his work " Die Formenreihe des Ammonites subradiatus, &c." (Geogn.-paheont. 

 Beitrage ii, Heft 2, b), for the express purpose of distinguishing variations in 

 time, to which alone this term is applied, from contemporaneous variations 

 in space, for which the term variety was and is in universal use. Both are 

 •considered as modifications of a single specific form which, together with all 

 its varieties and mutations, is termed a collective species (Collectivart). 

 The time-series alone, that is to say, the original or root-form with its succes- 

 sive mutations, constitutes the Formenreihe, a term subsequently translated 

 by Waagen himself as developmental series (Palasont. Indica, Jurassic 

 Fauna of Cutch, Cephalopoda, pt. 4, p. 238, 1875). It was the opinion of 

 Waagen that varieties were very variable and of little systematic value, 

 whereas mutations were, even in trivial characters, exceedingly constant, and 

 formed definite steps in evolution. Each mutation therefore might legiti- 



