﻿538 THE EVOLUTION OF ZAPHRENTIS DELANOUEI. [Nov. I9IO, 



been found to work satisfactorily for large zones, but the probable 

 error of the method forbade small subdivisions. 



A question of some theoretical interest, quite unessential how- 

 ever to the main issue of the paper, arose as to the nature of the 

 variation concerned in the formation of the genetic series that the 

 Author had established. The final demonstration of continuous 

 variation as a function of time seemed to demand an unbroken 

 sequence of identical deposits, containing, throughout, abundant 

 representation of the gens under examination. These conditions 

 were satisfied by the rocks in which Waagen and Dr. Eowe demon- 

 strated continuity of variation, but the Scottish sequence could not 

 be said to fulfil the same requirements. 



Dr. E. A . Bather, considering the paper from the standpoint of 

 a palaeontologist rather than from that of a zonal stratigrapher, 

 regarded it as important for the following reasons : — (1) It offered 

 a clear example of the most gradual kind of mutation, as opposed 

 to saltation ; even the form from the Lawston Limestones, though 

 called a ' sport ' by the Author, seemed to have arisen quite 

 gradually. Although there was no break between the extremes of 

 the series, still the necessity of having names justified the Author 

 in drawing arbitrary lines of division. (2) The assumption that 

 the type dominant in a lower bed was nearer the ancestor than 

 was the type dominant in a higher bed, an assumption hitherto 

 supported mainly by a priori considerations, was greatly strength- 

 ened by the proof of this particular instance. (3) While compar- 

 able to Dr. A. W. Howe's classic paper on Micraster. the present 

 communication upset the metaphysical and (to the speaker) unin- 

 telligible conception of an ' horizonal influence.' Different stages 

 of development co-existed at the same horizon in a single area, 

 while the same stage came in at different horizons in separate areas. 

 Physiological processes and needs doubtless produced changes of 

 organization more or less adapted to a changing environment, and 

 so originating new species through natural selection ; but what 

 either the internal or the external factors might be in the case of 

 the Scottish Zaplirentis, they hoped to learn from the Author. 



Mr. E. E. L. Dixon, in adding his congratulations to those of 

 previous speakers and remarking on the great value of the Author's 

 work to both the stratigraphist and the evolutionist, regretted, in 

 connexion with an observation made by Dr. Bather, that in most 

 rock-sequences the forms which were dominant at successive 

 horizons were not genetically connected. 



The Author, in reply to Dr. Vaughan, said that he considered 

 that the employment of the word ' mutation ' in the paper was 

 certainly in accordance with "Waagen's use of the term. He agreed 

 with Dr. Bather as to the in advisability of referring to the form 

 from the Lawston Limestones as a 'sport,' if by that term individual 

 abnormalities were implied. He regarded the species as a lateral 

 branch of narrow vertical range, although having a considerable 

 horizontal distribution. In conclusion, the Author expressed his 

 gratitude to the Fellows present, and especially to those who had 

 spoken in the discussion, for their cordial reception of the paper. 



