individual nautilus in the course of its development, he con- 

 sidered that there could be no doubt that they were justified 

 in the hypothesis that the causes at work were the same in 

 both cases, and that the inherent faculty, or power, or what- 

 ever else it might be called, which determined the successive 

 changes of the nautilus after it had been hatched, had been 

 operative throughout the whole continuous series of existence 

 of the genus from its earliest appearances in the later Silurian 

 rock up to the present day/^ 



This was his case for evolution, which he rested wholly 

 upon arguments of the kind he had adduced. 



Will it surprise you to be told, after this, that not only is 

 the argument hypothetical, but the facts are hypothetical too ? 

 for in the British rocks, and presumably elsewhere, the 

 orthoceras never turned into a cyrtoceras, for the simple and 

 sufficient reason, that the latter actually preceded the former. 



They both appear in the same geological day, the epoch of 

 the upper Cambrian, but the cyrtoceras is the first in the field.* 

 After their first appearances both subsist, fully formed and 

 equipped for the campaign of life, both preserving their 

 respective identities, quite distinct from each other, both 

 subsequently become scarce, and disappear. Whilst they 

 lived together side by side in the Silurian times, new genera 

 and species were added to each until there came to be no less 

 than 143 distinct creatures, going down from age to age in 

 lineal descent belonging to the orthoceras group, and 369 

 belonging to the cyrtoceras, enjoying the same surroundings 

 in every respect, but each species keeping to its own 

 model. 



Professor Huxley accounts for the multiplication and variety 

 of these creatures by the hypothesis that the cyrtoceras is an 

 orthoceras in the first instance curved by accident or by 

 external conditions, that thenceforward this individual pro-, 

 duced progeny similarly curved, 'and then similar causes 

 produced like occurrences in succession until the thousand 

 varieties of cephalopodous life thus arose, and what occurred 

 in one group happened also in all, and hence the variety 

 displayed throughout the animal kingdom. Now, whatever else 

 may have been the true history of the origin of the great 



* Salter's appendix : Memoirs of Geol. Survey, vol. iii., p. 358. " It 

 is the earliest of the Cephalopods known, and it is not a little remarkable 

 that the first species we meet with in ascending order should be — not ortho- 

 teras, which is the most diffused and persistent form, but a genus which, so 

 far as we know, is only Silnrian and Devonian." 



R 9. 



