DEVELOPMENT AND LACTATION OF MAMMALIA. 93 



bare, and whose body, instead of resembling that of a Dog, a 

 Horse, or any ordinary Mammal, presents the form belonging to 

 Fishes. Such are the Porpoise, the Whale, and other Mammals 

 of the order Cetacea, which are commonly, but erroneously, 

 reckoned as Fishes. ( 25). 



73. Development and Lactation. That which is most dis- 

 tinctive of the Mammalia, is their mode of development and of 

 nourishment during the earliest period of life. These animals 

 are all viviparous ; and whilst in an embryo condition, their life 

 is not supported by their own store of nutritive matter, as we see 

 is the case amongst oviparous animals, but they derive their 

 nourishment direct from the blood of the mother; and, after 

 birth, the young still live at her expense, as she suckles them for 

 a longer or shorter time. The milk, destined for this purpose, is 

 a white and opaque liquid, composed of water, holding in 

 solution sugar, casein, salt, and a little free lactic acid; and 

 holding in suspension some globules of butter. Its qualities vary 

 a little amongst different animals, and may be modified by the 

 food of which they make use. In general it leaves, by evapora- 

 tion, from ten to twelve per cent, of solid parts ; but its richness 

 may vary much according to circumstances. This nutritious 

 liquid is secreted by particular glands, named mammary; which 

 exist in both sexes ; but which only serve for the support of the 

 young, amongst the female. Analogous organs are not to be met 

 with in any other class of the Animal Kingdom ; and it is from 

 their existence, amongst all the animals composing the group 

 whose history we are commencing, that Zoologists have given to 

 these beings the name of MAMMALIA. The number of the mam- 

 mary glands is in general nearly in proportion to that of the 

 young produced at each birth. There are often only two (amongst 

 the Monkeys, the Elephant, the Goat, and the Horse, for ex- 

 ample) ; but sometimes, also, their number is much more con- 

 siderable. Thus, the Cow, the Stag, the Lion, have four ; the 

 Cat, eight; the Pig and the Rabbit, ten; the Bat, ten or twelve; 

 and the Agouti, twelve or fourteen. The position of these glands 

 varies also. Amongst the Monkeys and Bats they are placed 

 under the thorax, as in Man ; in the greatest number of Carni- 



