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, anda 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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