GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



19 



whose best-known representatives are the 

 Ichthyosaurus and Plesiosaurus. 



On the other hand, we can point to char- 

 acters which are plainly the result of a retro- 

 grade development. The small bones found 

 in the pelvic region in many cetaceans mani- 

 festly show that the ancestors of the whales 

 had hind -limbs, which became degraded 

 through a process of special adaptation, and 

 finally disappeared. In all the series of 

 extinct and living mammals and reptiles yet 

 known we cannot point to any type to 

 which the whales could be attached without 

 violence. The Zeuglodonts, large marine 

 forms, with hind -limbs, belonging to the 

 upper Miocene and the Pliocene, unite cer- 

 tain characters of the whales with a den- 

 tition similar to that of the seals, but it is 

 impossible to regard them as representing 

 the primitive type of the whales, which are 

 contemporary with them or even precede 

 them. The diffuse placenta connects the 

 cetaceans with the ungulates. If it is possible 

 to connect the other placental mammals in 

 more or less direct lines of succession with 

 the old dwarf mammalian types, whether 



marsupial or otherwise, which have been 

 discovered in Triassic and Jurassic strata, 

 such an attempt would be altogether impos- 

 sible with the whales. How could animals 

 with a reptilian dentition be derived from 

 old stems which already possessed a speci- 

 alized dentition and molars with double 

 roots ! 



All these questions are insoluble in the 

 present state of our knowledge. The balance 

 inclines at present, perhaps, in favour of a 

 direct connection of the whales with the 

 extinct sea-lizards, the Enaliosaurs, on the 

 one hand, and the Mosasaurs and Clidastes 

 of the Chalk, on the other, although all these 

 have only a single joint-surface at the back 

 of the head (a single occipital condyle), while 

 the whales, like all mammals, have two. If 

 this surmise should be confirmed, it would 

 furnish a beautiful proof of the evolution of 

 the class of the Mammalia from various 

 stocks. But for the present these are only 

 doubtful surmises, which, nevertheless, are 

 better supported than those which would 

 derive the whales either from the seals or 

 from the ungulates. 



