1^8 R. D. Oldham— Some Bough Notes for the [No. 3, 



recent past has affected both hemispheres of the globe, but, as there is 

 reason to believe that such have occurred at various periods in the 

 history of the earth, we are dependent on the otherwise less accurate 

 palaeontological evidence for determining whether the strata shewing 

 signs of glacial action can have been deposited at the same period or 

 must belong to widely separated geological epochs. 



There can be no doubt that of all forms of paljBontological evidence 

 the most trustworthy is that afforded by the marine moUusca. Inha- 

 biting as they do an element of more uniform temperature, and of 

 which every part is in continuous if circuitous connection with the rest, 

 it is but natural that they should be more uniform in character than the 

 fauna of the land, while the simplicity of their structure, greater than 

 that found among vertebrates or higher invertebrates, renders them less 

 liable to change through alteration of the conditions under which they 

 live. On the other hand, this very stability of organism renders them 

 useless for the exact correlation of strata far separated from each other ; 

 for mere determination of homotaxy, even did this exist in the sense in 

 which the term was originally intended to bear, would be but of little value 

 to the physical geologist, to whom the terms ' Jurassic ' or ' Carbonifer- 

 ous,' if determined merely on palasontological grounds, are as meaningless, 

 for determination of dates in the history of the earth, as the analogous 

 terms ' Stone Age ' and ' Bronze Age ' are for determining periods in 

 the history of the human race. 



But, if the evidence afforded by marine mollusca is not sufficiently 

 accurate and trustworthy, how much more is this true of that afforded 

 by the terrestrial fauna and flora. True, the duration of the existence of 

 a species of cycad, conifer, and, possibly, even a fern may be shorter on 

 the average than that of a species of mollusc, and to this extent it may 

 be a more accurate index of contemporaneity ; but it is comparatively 

 seldom that identical species are found in far separated deposits, and 

 palaeontologists have consequently to depend mainly on what are called 

 ' allied species.' Now the hard parts of animals, which in almost every 

 case are all that are preserved to us, give, for the most part, a very 

 true and real indication of the affinity of the animal to which they 

 belonged, while, from the leaf of a tree or the frond — generally barren 

 or with the fructification obliterated in fossilization — of a fern, little or 

 nothing can be gleaned of the relationship of the plant to which it origi- 

 nally belonged ; thus no one would doubt that two specimens of Terehra- 

 tula or Ammonite, declared by a competent palaeontologist to belong to 

 the same species, would, if we could recover their soft parts, still prove to 

 belong to the same or very closely allied species, while, on the other hand, 

 we have lately been informed, by a palaeobotanist whose competence none 



