EXTERNAL EARS 



the way in which they develop in the embryo, can alone 

 settle the fact that they are not hairs, but true, though 

 somewhat rudimentary, feathers. But no practical 

 difficulty arises in this case ; for on other parts of the 

 body are plenty of obvious feathers which no mammal 

 ever possesses. 



A third feature is also absolutely distinctive, and that 

 is the presence in the female, with rudiments in the male, 

 of cutaneous glands which secrete milk for the nourish- 

 ment of the young when born. No other vertebrate 

 possesses anything even remotely resembling the mam- 

 mary glands of the mammalia. 



A rash observer might say that mammals are never 

 scaly, while reptiles, fishes, and even birds (their feet) 

 are always so. It is difficult, however, to distinguish 

 accurately the scales upon a rat's tail from those of a 

 lizard, though it is true that the scales of the pangolin, 

 or scaly ant-eater of Africa and India, are not real scales, 

 but merely agglutinated hairs. 



No one can look at the head of a mammal, that is of 

 course of a terrestrial mammal (for here again, as in so 

 many features, the whales are exceptional), without ob- 

 serving how impossible it is to mistake that group for 

 any of the lower lying groups of vertebrate animals. 

 With a few exceptions such as the seals as well as the 

 whales already mentioned and a small selection of other 

 mammals external ears are present in that group, and 

 often of conspicuous size. In the lizards, frogs and birds 

 the external ear is absent, and there is merely the external 

 auditory passage visible, covered by the tympanic 

 membrane ; and even that is not always present. Coupled 

 with the presence of ears is a look of alertness, quite dis- 

 tinctive of the mammalia, and a more fidgetty demeanour 

 than that of lower vertebrates, except of course birds. 

 There is but little torpidity among mammals save in 

 those few cases, such as the hedeghog, where the animal 



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