May 1902.] BOTANICAL SOCIETY OF EDINBURGH 241 



of course, implies the existence, within the drainage area 

 of the river, of a luxuriant vegetation, and much of it. 

 (2) The number of extinct species. This clearly brings 

 to our notice the fact that mollusca, and also plants, have 

 longer lives as species than have mammalia — a fact of 

 considerable importance in connection with many geo- 

 logical questions. (3) The presence, along with the 

 forms of mammalian life characteristic of temperate 

 regions, of a distinctly sub-tropical fauna.^ (4) The 

 most important fact of all, that certain distinctly arctic 

 mammals w^ere living within the basin of the Fihine at 

 this period. I would refer especially to such animals as 

 the Glutton and the Musk-Sheep, not to speak of others. 

 Unless we are to assume that the habits of these animals 

 are different now from what they were then, we are 

 driven, it seems to me, to the conclusion that an outward 

 migration from the colder regions of the north was already 

 in progress, and had extended as far south as some of the 

 northern parts of the Ehine basin. We have seen that the 

 history of the marine mollusca in still earlier Pleiocene 

 times had foreshadowed this event. What I want to 

 suggest, in referring to these matters, is that, at the time 

 under consideration, rather more snow was falling on 

 Scandinavia than the summer's warmth there sufficed to 

 melt ; and that, as time went on, the quantity left each 

 summer, even on the lowlands, gradually became larger. 

 An exodus of the fauna, and, necessarily, also of the flora, 

 therefore gradually set in. Plants slowly retreated to 

 what, for the time being, were to them more congenial 

 habitats ; and, as we shall see presently, it was a very 

 long time before they returned. 



At this point we may, not unprofitably, turn to the 

 consideration of the age of these events, as measured by 

 ordinary chronological standards. One cannot, of course, 

 be sure of every point in the argument that follows ; but 

 still there are some reliable facts to go upon. It is one 

 of the first principles in Geology, and especially when 

 we are dealing with the later Tertiary rocks, that the 

 contemporaneity, or otherwise, of two given sets of rocks 



1 These may have drifted from the warmer .loulhern parts of the 

 basin of the Rhine. 



