292 TOWARDS THE HUMAN FORM 



except in the water, but surpassing them greatly in both 

 agility and intelligence. Like the Reptiles, some are 

 carnivorous, others herbivorous. The carnivores are planti- 

 grade or digitigrade, without presenting any great modifica- 

 tions in limb ; the herbivorous types not only raised them- 

 selves upon their digits, but succeeded in achieving what 

 the Reptiles never did, a stance on the end of the distal phalange, 

 around which a nail developed so as to form a hoof ; they now 

 became " unguligrade " and constituted the order of Ungulata. 

 As the Reptiles had done before them, they took possession 

 of the air and of the water. We saw before (p. 193) how climbing 

 Mammals of many orders acquired a parachute, and how they 

 led up to the Bat, which has wings constructed on the type 

 of the Pterosaurs of the Secondary era, but more highly 

 perfected, since four of its digits instead of one are employed 

 in supporting the flying-membrane. Indeed, this seems to 

 have been achieved twice, that is to say, by two distinct 

 types of Mammals, for the large fruit-eating tailless Pteropus 

 of warm countries is very different from the ordinary Bat, 

 which is insectivorous and has a long tail incorporated in the 

 wing membrane. 



Twice, also, have placental Mammals managed to acquire 

 the freedom of the ocean, as did the Ichthyosaurians, which 

 they resemble but surpass in the perfection of their adaptations. 

 Thus we find Herbivorous Mammals constituting the order 

 of Sirenidia, which preserved the mobility of their elbow, 

 and Carnivorous Mammals the order of Cetacea, which preserved 

 only the mobility of the shoulder. In both cases the 

 hind limbs disappeared, and the tail became a very powerful 

 motor organ. The Sirenidians have pectoral mammae and rise 

 half out of the water to suckle their young ; the Cetaceans, by 

 a sudden muscular contraction, ejaculate their milk into 

 the mouths of their young, which do not suck : their mammae 

 are inguinal. Possibly a third type presents this same 

 adaptation to an aquatic life. Seals are Carnivora of an 

 advanced type which have preserved their four limbs in a form 

 less removed from the ordinary foot than the natatory paddles 

 of the Sirendia and the Cetacea, modelled on those of the 

 Ichthyosaurians. However, in the Eogene Period there 

 lived in Alabama and New Zealand a huge aquatic Mammal, 

 the Zeuglodon, which sometimes attained a length of thirty 



