LIFE IN TERTIARY TIMES 293 



metres, like our Baleen Whales and Cachalots. It had no hind 

 limbs and its tail was probably pointed, not flat like that of the 

 Cetacea, and it had molar teeth with two roots, singularly 

 recalling those of Seals, whereas in Cetacea all the teeth 

 are alike and have but one root. 



We have now to see how these diverse forms were grouped 

 during the successive periods of the Tertiary Epoch. 



The laws determining modification of limbs are simple 

 and precise. Confining our attention to the exterior aspects, 

 we may say that, whatever diet was adopted, every series 

 began by forms in which the feet rested entirely on the ground, 

 consequently known as plantigrade. Then the foot gradually 

 became raised so that the toes alone were on the ground ; 

 in these conditions the shortest toes soon ceased to touch 

 the ground ; being unused, they tended to disappear : but 

 this disappearance occurred much earlier in a hind- than in 

 a fore-limb. The anterior limb was often put to various 

 uses, whereas the hind-limb was always more specialized 

 for the locomotor function : it was the main instrument of 

 propulsion in other types besides Mammals. So much so that 

 the following rule may be formulated : The hind-legs of land 

 quadrupeds, more especially utilized for propulsion, are both 

 more highly developed and more modified than the fore-legs. 

 In general the contrary is true for aquatic Vertebrates, for 

 as the tail plays a considerable part in propulsion, the unused 

 posterior limbs are either reduced (sub-brachian Fishes, 

 Ichthyosaurians) or disappear (Sirenia, Cetacea, Siren among 

 the Batrachians, Sirens, Eels, etc.). 



The limbs of the Carnivora undergo no important modifica- 

 tions. The plantigrade forms are numerous and preserve 

 the digits on all four legs. In the digitigrade forms, the Dog and 

 Cat have only four digits on their hind and five on their fore 

 feet. Hyenas have only four digits on all four feet. In the 

 Herbivora, modification extends much further. The digits 

 of the Carnivora, utilized more or less for seizing and holding 

 prey, terminate in claws. It is the same with Insectivora, 

 whose feet remain pentadactyl. In the Rodents, on the 

 contrary, the number of toes is often reduced to four or even 

 three, and the structure of the tridactyl hind foot of the 

 Jerboa, with its united metatarsals, recalls the foot of a Bird. 



Certain Rodents (Cavy or Cabiai) have the extremities 



