﻿lxxvi PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May I905, 



outward form of these genera is not due, however, to a very close 

 genetic relationship ; and whereas Azygography is characteristic of 

 early Ordovician strata, Monograptus is confined to those of Silurian 

 age. Similar cases of non-contemporaneous homoeomorphic forms 

 have been noted among the ammonites and the brachiopods, and 

 will probably be detected in the future among fossils belonging ta 

 other zoological groups. 



Turning now to mutations, where the varieties which succeed 

 one another are closely related genetically, we have to consider the 

 necessity for very careful study in distinguishing one mutation 

 from another, as proved by recent work. In the early days of 

 palseontological investigation, the characters utilized for the purpose 

 of separating varieties were sufficiently obvious to be detected in 

 .single specimens, and the minute variations which have in some 

 cases been found to mark off mutations from each other were un- 

 known. The researches of Mr. W. Bateson on the forms of Cardium 

 in the Aralo-Caspian basin, and of Dr. A. W. Eowe on the variations 

 of Micraster at different horizons, have proved the value of dealing 

 with suites of specimens of each mutation, and subjecting them ta 

 minute and accurate measurement. It is obvious that much work 

 of a similar nature remains to be accomplished in the case of other 

 fossil forms ; and those who follow Dr. Eowe's line of research 

 will naturally undertake the study of forms which are preserved in 

 sufficient numbers in the fossil state. This cannot be done by the 

 examination of museum-specimens ; but specialists in the different 

 groups of organisms must, as Dr. Eowe has done, obtain the 

 specimens for themselves from the rocks in which they are 

 entombed. 



Another matter too often overlooked is the frequency of occur- 

 rence of individuals in a particular stratum or group of strata, and 

 especially the relative proportions of different species in the strata. 

 Sometimes forms are stated to be common or rare, and occasionally 

 a more detailed division into ' very rare, rare, common, and very 

 common ' is made. But it is very desirable that such groups of 

 strata should be studied as contain a varied fauna composed of 

 numerous individuals of easily-identified forms, and that the pro- 

 portions of the different forms which occur at different horizons 

 should be, as far as possible, ascertained. The rich graptolite- 

 bearing beds of the Moffat district, or those of the Llandovery age 

 in Lakeland, would be well adapted for this purpose ; and a compa- 

 rison of the strata of two or three sections situated at some distance 



