C— GEOLOGY 83 



then proterogenesis of one feature and tachygenesis of another are pro- 

 ceeding simultaneously in one and the same series of organisms. 



A second series, viz. Rastrites peregrinus-Monograptus urceolus, is 

 retrogressive and illustrates the second half of Miss EUes's statement. 

 In it the primitive closely approximated triangular type of thecas re- 

 appear deuterogenetically at the distal end of the stipe, and in later 

 members of the series they extend tachygenetically to early and yet earlier 

 stages of the development of these. In association with this the less 

 primitive tubular and isolated thecae are gradually eliminated until only 

 four or five are to be found at the proximal end. 



Though in these two examples proterogenesis and tachygenesis happen 

 to coincide with changes hitherto described as anagenetic and catagenetic 

 it must not be supposed that these are synonymous terms, for in other 

 cases the coincidence is reversed. Thus, for example, in the Liparo- 

 ceratidce the passage from the inflated whorls of Liparoceras to the much 

 thinner whorls of the ' Capricorn ' would usually be regarded as an 

 example of catagenesis, nevertheless the origin and extension of the 

 Capricorn condition provides, as we have already seen, a typical example 

 of coenogenesis and proterogenesis. It is difficult to refrain from ex- 

 pressing the hope that Spath will now work out for us the other half of 

 his story, to wit, the origin of Liparoceras itself. I am inclined to suspect 

 that will it provide us with a good example of deuterogenesis and tachy- 

 genesis. But when we begin to hope, to suspect, and to prophesy, it is 

 a sign that the springs of knowledge are drying up and that it is time to 

 cease talking. 



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