Missing Chapters of Geological History. 13 



arrive ; and in his addresses to the G-eological Society on the 

 subject of the breaks in succession of the British strata, Pro- 

 fessor Ramsay was justified in supposing that his hearers would 

 at once understand and be willing to admit whatever conclu- 

 sions a fair consideration of the recorded facts on the subject 

 might justify. 



But in addition to the technical argument, presented in an 

 abstract form by tabular documents, prepared for the occasion 

 and since published, there is a more general, though not less 

 important view that it is very desirable to set forth. 



It. will be evident that some definition of species, and of 

 specified difference in reference to extinct species, must be 

 adopted for the occasion of this discussion. Even if it be the 

 case that species are derived one from another, there yet exists 

 between any two recognized species a real difference and a 

 marked departure from the normal and typical character. This 

 we may regard as the practical equivalent to the essential and 

 underivable difference assumed to exist by those who look on 

 species as specially created and incapable of change. For the 

 sake of convenience naturalists must continue to speak of 

 species and regard them as distinct, even if the old notions be 

 subverted and Darwinian views reign triumphant. On the 

 other hand, if the reader be convinced that the derivation of 

 species by natural means is not the habit of nature, he cannot 

 deny, and will no doubt very readily admit, that there are many 

 cases' of strong analogy or resemblance between what he feels 

 bound to regard as distinct species, and many others in which 

 an affinity or relationship exists extremely puzzling and difficult 

 to explain on any recognized anti-Darwinian theory. And this 

 is so even where no specific identity of the ordinary kind seems 

 possible. 



If it be granted that certain points of difference in the struc- 

 ture and form of the skeleton or the shell of an animal, or of 

 the corresponding parts in vegetable structures, really mark 

 specific character or permanent varieties in all the principal 

 divisions of the animal and vegetable kingdoms, we are justified 

 in examining the fossils of the great series of stratified rocks 

 with a view to comparison. If we find a group of remains 

 (animal or vegetable) crowded in certain rocks, and absent from 

 overlying or underlying rocks of similar mineral character, we 

 may reasonably seek for an explanation of this in altered phy- 

 sical conditions. Either the depth of deposit has changed — 

 what was once deep, being now shallow — or the converse ; or 

 a difference in the state of the water — salt water replaced by 

 fresh, clear water by muddy, or the converse — has taken place. 

 If we find that in examining successive strata there are 

 always differences in the groups of organic remains — that 



