C— GEOLOGY 79 



the coiling manifested in late forms of the gens Grypheea incurva were 

 anticipated in late stages of growth in the earlier forms. 



' Mutation.' 



Fenton in the work already mentioned describes a very large number 

 of new species, sub-species and ' forms,' and provides numerous diagrams 

 and tables which show their morphological relationships, their times of 

 appearance and their ranges in time. An inspection of these reveals the 

 fact that, whilst in some groups the new types appear in sequence at 

 relatively wide intervals of time, in others they come on rapidly, and in 

 yet others they appear almost if not quite simultaneously. In this last 

 case he suggests that they may have arisen by ' mutation ' in the De 

 Vriesian sense. He finds difficulty, however, in definitely asserting this 

 to be the case because the group of new types may be arranged in a 

 continuous evolutionary series. Personally I feel no difficulty in believing 

 in the simultaneous appearance, in a large population, of types which 

 fit into a series, for such could be regarded as one more phase or degree 



a b. c. d. e f 



Fig. 4. — Diagram showing a series of clymenid ammonoids. a-c, Kampto- 

 clymenia. d, Triaclymenia. e, f, Parawocklumeria. (From Schindewolf .) 



of rapidity in production. There are not wanting facts which indicate 

 that this simultaneous or sub-simultaneous appearance may be more 

 common than is generally realised. Several examples may now be 

 mentioned. 



Among the examples quoted by Schindewolf in support of the principle 

 of proterogenesis is one drawn from the cephalopod family the Clymenidee, 

 which lived during the Devonian period. It consists of a number of 

 genera and species in which, at one end of the series, the shell has the 

 normal type of spiral coil (Fig. 4) with an almost circular outline through- 

 out development. In the next member of the series the innermost 

 portion of the spiral has a triangular outline. In other members of the 

 series the latter form of outline finds every degree of expression up to 

 one in which it prevails at all stages of growth, including the adult. The 

 series as it stands may be quoted in support of either the proterogenetic 

 or the tachygenetic view, according to which end of the series is taken as 

 the starting point. Schindewolf adopts the former. It is natural to 

 look to stratigraphical evidence as the adjudicator between the two, but 

 according to Schindewolf's account, all these grades appear simultaneously 

 in the lowest stratum ; and the simpler forms, that is to say those in 

 which the triangular outline is exhibited only in the innermost whorls, 

 are progressively eliminated until in the uppermost strata only those 



