LINNEAN SOCIETY OF LONDOK. ^;^ 



of tlie gigantic rhinoceros like Bahichithcrinm and its allies in 

 central Asia are also notew ortliy in the same connection. 



If, however, there seems to be some hope of discovering the 

 beginning of the various lineages when exploration has proceeded 

 further, the difficulty of explaining tlieir end appears to be as 

 great as ever. As soon as they have attained their widest 

 geographical range, have become perfectly well adapted to their 

 environment, and are represented by the largest individuals, they 

 begin to show signs of decline and disappear at least as rapidly as 

 they originated. Indeed, in a moditied sense, Cuvier's early 

 tlieory of the successive " revolutions of the globe," which have 

 culminated in the world of life as it now exists, is still distinctly 

 plausible. The ancestors of the crustaceans and arachnids, for 

 example, when tiiey held the foremost place at the beginning of 

 the Devonian period, attained their largest size — some with a body 

 six feet in lengtli — just before the dominance of fishes, which 

 at first were comparative dwarfs. Many of the reptiles, too, 

 immediately before they lost their dominant position at the end 

 of the Cretaceous period, were among the largest animals that 

 ever lived ; and when these disappeared, there was for a long 

 period among the mammals which replaced I hem no animal larger 

 than a sheep. The alternating hixuriance and poverty in tlie 

 development of life in successive phases is indeed striking, and at 

 present baffles explanation. 



Most remarkable, perhaps, is the fact that we rarely find any 

 evidence of direct competition between the flourishing type that 

 is doomed and the incipient higher type that is destined to 

 replace it. Tor example, so far as we know, the Ichthyosaurians, 

 Mosasaurians, and other giant sea-reptiles, which ranged in 

 abundance through all seas from the Arctic regions to New 

 Zealand, never came in contact with the whales and porpoises 

 which were eventually to take their place. Fossils already dis- 

 covered in Egypt suggest that the toothed whales originated from 

 primitive land-carnivores after the old sea-reptiles had tlisappeared. 

 When, however, this new race passed into the vacant seas, it soon 

 nuiltiplied and spread widely ; and by the end of the Eocene 

 period there were toothed whales {ZevyJodon) some 70 feet in 

 length. 



Tlie solution of the problems suggested by these various facts 

 is delayed and rendered all the more difliciilt by the astonishing 

 uniformity in the geographical distribution of life in past ages. 

 AVhen the corresponding fossil-bearing rocks in different parts of 

 the world are explored, there is as a general rule very little difter- 

 ence in their contained faunas and floras. When Ameghino, for 

 instance, first found reptiles and fishes in a Jurassic stratum in 

 Patagonia, he sent me a skull of a sea-crocodile {Metriorhynchvs) 

 and a skull of a predaceous ganoid fish (Bi/psoconmis), such as 

 might have been found similarly associated in the Middle Jurassic 

 Oxford Clay at Peterborough in England. When I received the 



LINN. SOC, PKOCEEDINGS. — SESSION 1922-23. cl 



