Septembee 10, 1909] 



SGIENCi: 



327 



able elongation of the body and a fringe- 

 like extension of the fins. Among higher 

 fishes, too, there are numerous instances of 

 the same phenomenon, but in most of these 

 the ancestors still remain undiscovered, and 

 it would thus be tedious to discuss them. 



Finally, in connection with these obvious 

 symptoms of old age in races, it is interest- 

 ing to refer to a few strange cases of the 

 rapid disappearance of whole orders of ani- 

 mals, which had a practically world-wide 

 distribution at the time when the end came. 

 Local extinction, or the disappearance of 

 a group of restricted geographical range, 

 may be explained by accidents of many 

 kinds; but contemporaneous universal ex- 

 tinction of widely spread groups, which 

 are apparently not affected by any new 

 competitors, is not so easily understood. 

 The Dinosaurs, for instance, are known to 

 have lived in nearly all lands until the close 

 of the Cretaceous period ; and, except per- 

 haps in Patagonia, they were always ac- 

 companied until the end by a typically 

 Mesozoic fauna. Their remains are abun- 

 dant in the Wealden formation of western 

 Europe, the deposit of a river which must 

 have drained a great continent at the be- 

 ginning of the Cretaceous period; they 

 have also been found in a corresponding 

 formation which covers a large area in the 

 state of Bahia, in Brazil. They occur in 

 great numbers in the fresh-water Upper 

 Cretaceous Laramie deposits of western 

 North America, and also in a similar for- 

 mation of equally late date In Transyl- 

 vania, southeast Europe. In only two of 

 these regions (southeast England and west 

 North America) have any traces of mam- 

 mals been found, and they are extremely 

 rare fragments of animals as small as rats ; 

 so there is no reason to suppose that the 

 Dinosaurs suffered in the least from any 

 struggle with warm-blooded competitors. 

 Even in Patagonia, where the associated 

 mammal-remains belong to slightly larger 



and more modern animals, these fossils are 

 also rare, and there is nothing to suggest 

 competition. The race of Dinosaurs seems, 

 therefore, to have died a natural death. 

 The same may be said of the marine rep- 

 tiles of the orders lehthyosauria, Plesio- 

 sauria and Mosasauria. They had a prac- 

 tically world-wide distribution in the seas 

 of the Cretaceous period, and the Mosa- 

 sauria especially must have been extremely 

 abundant and flourishing. Nevertheless, 

 at the end of Cretaceous times they disap- 

 peared everywhere, and there was abso- 

 lutely nothing to take their place until the 

 latter part of the Eocene period, when 

 whales and porpoises began to play exactly 

 the same part. So far as we know, the 

 higher race never even came in contact with 

 the lower race ; the marine mammals found 

 the seas vacant, except for a few turtles 

 and for one curious Rhynchocephalian 

 reptile (Champsosaurus) , which did not 

 long survive. Another illustration of the 

 same phenomenon is probably afforded by 

 the primitive Carnivora (the so-called 

 Sparassodonta), which were numerous in 

 South America in the Lower Tertiary 

 periods. They were animals with a brain 

 as small as that of the thylacines and 

 dasyures which now live in Tasmania. 

 They appear to have died out completely 

 before they were replaced by the cats, 

 saber-toothed tigers and dogs, which came 

 down south from North America over the 

 newly emerged Isthmus of Panama at the 

 close of the Pliocene period. At least, the 

 remains of these old carnivores and their 

 immigrant successors have never yet been 

 found associated in any geological forma- 

 tion. 



These various considerations lead me to 

 think that there is also deep significance in 

 the tendency towards fixity in the number 

 and regularitj^ (or symmetry) in the ar- 

 rangement of their multiple parts, which 

 we frequently observe in groups of animals 



