﻿BRITISH FOSSIL CEPHALOPODA. 241 



obscure. When our ignorance of the actual course of development, on the supposition 

 of its having taken place, is duly allowed for, none of the facts observable in the 

 present study, at least, make in any way against the general theory of evolution. 

 We find it impossible to say that any one position of the siphuncle, or one style of 

 ornament, indicates an advance upon any other, and the assumption of such being 

 the case may easily be made the basis of apparent contradictions. But while the 

 general history has been one of advance, we have no right to expect the same to be 

 obvious when we come to smaller groups. Yet, taking the four groups into which 

 I have divided the Nautiloidea — the Conici, the Spirales, the Inflati, and the 

 Irregulares — of the first two, which may be considered the simpler, 44 per cent, 

 occur in the Lower Silurian ; while of the two latter, which are more specialised, 

 only 18 per cent, are found in the earlier rocks. So also, among the Orthocerata, 

 the Lower Silurian forms are either for the most part the simpler, or have large 

 siphuncles, which is exactly the opposite to the final character of that organ in the 

 class. So, too, amongst Nautilus the earliest forms have an internal siphuncle, while 

 the opposite position is the finally dominant one in the class. The age of the genus 

 Ascoceras also, when that is rightly placed from purely zoological considerations, is 

 not a difficulty in the way of evolution, as Barrande supposed it, but it occupies its 

 natural place. 



To the general theory of evolution, therefore, which merely states that every 

 form of life has been developed from a preceding one nearly allied to it, the present 

 study affords no contradiction or difficulty, but affords aid, if not as great as could 

 be desired, yet as much as could be expected. 



With regard to any particular method of evolution, such as that known as 

 " natural selection," the case is different. We have under view in the present 

 study a fair representative of successive fauna of the same class, and the history of 

 the class must at least be feebly written here. Yet while we do not find the forms 

 rigidly bound within " specific " limits, they do not appear to be scattered haphazard 

 over the whole range of possible varieties, but are remarkably grouped round a 

 series of central types. Now, so long as the environment remains constant, the 

 process of evolution by indefinite variation and survival of the fittest should either 

 be uniform and leave relics having no special grouping, or it should cease when the 

 best adaptation to the environment had been acquired. In the first case, the 

 arrangement of the fossils in even nominal " species " would never have been sug- 

 gested. In the second case, all variety must depend on change of environment. 

 If this change, again, had been approximately uniform, the same result as in the 

 first case would be brought about ; and if it had been sufficiently rapid to produce a 

 species, all the forms of life must have been more or less affected by it, and such 

 changes ought to be marked by an extensive divergence of fauna. Such is not 



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