416 Notices of Memoirs — British Association — 



As to tlie beginning of the birds, it can only be said that towards 

 the end of the Triassic period there arose a race of small Dinosaurs 

 of the lightest possible build, exhibiting many features suggestive 

 of the avian skeleton ; so it is probable that this higher group alSo 

 originated from an intensely restless early community of reptiles, 

 in which all the variations were more or less in the right direction 

 for advancement. 



In short, it is evident that the progress of the backboned land 

 animals during the successive periods of geological time has not been 

 uniform and gradual, but has proceeded in a rhythmic manner. 

 There have been alternations of restless episodes which meant real 

 advance, with periods of comparative stability, duiing which the 

 predominant animals merely varied in response to their surroundings, or 

 degenerated, or gradually grew to a large size. There was no transition, 

 for instance, between the reptiles of the Cretaceous period and the 

 mammals which immediately took their place in the succeeding 

 Eocene period : those mammals, as- we have seen, had actually 

 originated long ages before, and had remained practically dormant 

 in some region which we have not yet discovered, waiting to burst 

 forth ia due time. During this retirement of the higher race the 

 reptiles themselves had enjoyed an extraordinary development and 

 adaptation to every possible mode of life in nearly all parts of the 

 globe. We do not understand the phenomenon — we cannot explain 

 it ; but it is as noticeable in the geological history of fishes as in 

 that of the land animals just considered. It seems to have been first 

 clearly observed by the distinguished American naturalist, the late 

 Professor Edward D. Cope, who termed the sudden fundamental 

 advances 'expression points', and saw in them a manifestation of 

 some inscrutable inherent ' bathmic force '. 



iSf a- i^ ^i i:- •!{• i^ 



The demonstration by fossils that many animals of the same 

 general shape and habit have originated two or three times, at two 

 or three successive periods, from two or three continually higher 

 grades of life, is very interesting. To have proved, for example, 

 that flying reptiles did not pass into birds or bats, that hoofed 

 Dinosaurs did not change into hoofed mammals, and that Ichthyosaurs 

 did not become porpoises, and to have shown that all these later animals 

 were mere mimics of their predecessors, originating independently 

 from a higher yet generalized stock, is a remarkable achievement. 

 Still more significant, however, is the discovery that towards the 

 end of their career through geological time totally different races of 

 animals repeatedly exhibit certain peculiar features, which can only 

 be described as infallible marks of old age. 



The growth to a relatively large size is one of these marks, as 

 we observe in the giant Pterodactyls of the Cretaceous period, the 

 colossal Dinosaurs of the Upper Jurassic and Cretaceous, and the 

 larger mammals of the Pleistocene and the present day. It is not, 

 of course, all the members of a race that increase in size ; some 

 remain small until the end, and they generally survive long after 

 the others are extinct ; but it is nevertheless a common rule that 

 the prosperous and typical representatives are successively larger. 



