THE MAMMALIA. 



those which have five digits on each limb have 

 undergone no modification, but have preserved 

 their original type. 



Comparison of Vertebrate Types: Ichthyop- 

 sida and Sauropsida. — The five-toed vertebrates 

 when compared with one another show some very 

 remarkable points of resemblance and difference. 

 The amphibians (frogs, toads, &c.) have a number 

 of characters in common with the fishes. Types 

 may be found in regard to which, apart from the 

 structure of the limbs, doubt might be entertained 

 as to whether we should refer them to the one class 

 or the other. These two classes have accordingly 

 been rightly comprehended in one larger group — 

 the Ichthyopsida, a group the members of which 

 breathe, sometimes during their whole life, some- 

 times only in the larval condition, through true 

 gills, which are never developed in the other three 

 classes. 



A number of common characters unite also the 

 reptiles and birds. As to the fact that the ancestors 

 of the birds were reptiles, that indeed birds are 

 only reptiles which in the course of geological 

 epochs have developed further in a particular 

 direction, probably no doubt can any longer be 

 entertained. The development of the embryos, 

 the structure of the envelopes of the ovum, the 

 peculiarities of the skeleton, w-hich we cannot here 

 enumerate, agree to such an extent, that the uniting 

 of these two classes, at the present day apparently 

 so different, into the single large group of the 

 Sauropsida is fully justified. For the sake of 

 those readers who are not initiated into these 

 investigations, I mention here two facts of great 

 importance: all the Sauropsida have only a single 

 articulating surface (in scientific language a single 

 articular condyle) at the back of the head, with 

 the aid of which the head is rotated on the first 

 vertebra, and all the earliest birds have teeth 

 exactly like those of reptiles. Even yet these teeth 

 sometimes appear in embryonic life, as in the parrots 

 and certain other birds, but they never emerge 

 through the gum, and are re-absorbed soon after 

 their appearance. 



Position of the Mammalia in the Sub-kingdom 

 Vertebrata — The Mammals accordingly form a 

 third large division, entirely distinct from the other 

 vertebrates. Formerly they were grouped along 

 with the birds as •' warm-blooded " animals, because 

 like these they have a uniform and tolerably high 

 temperature of the blood. Over against these two 

 classes were placed the reptiles, amphibian.s, and 



fishes with cold blood, or rather with blood the 

 temperature of which depends on that of tlie sur- 

 rounding medium. But naturalists have been com- 

 pelled to recognize the fact that this physiological 

 agreement has only a subordinate value. 



But if, on the one hand, the mammals form a 

 separate group, yet, on the other hand, it cannot 

 be gainsaid that in respect of certain characters 

 they present a surprising resemblance to the am- 

 phibians. Besides in the structure of the ovum 

 this resemblance is also shown in the fact that all 

 mammals have, like the amphibians, two lateral 

 condyles at the back of the head, and that the 

 movements of the head are altogether different 

 from those which are observed in reptiles and 

 birds. Let us take by way of illustration the 

 movements of the human head. When we raise 

 or depress the head, that is, whenever we move 

 it vertically, we allow the condyles to play upon 

 the corresponding articular surfaces of the first 

 neck vertebra; but when we rotate the head hori- 

 zontally, then we cause the first neck vertebra to 

 turn round an upright process of the second. The 

 joints for these two sorts of movements are accord- 

 ingly different. The birds, on the other hand, 

 execute their movements of the head in all direc- 

 tions by means of the single median condyle; they 

 have no second joint of the kind just spoken of. 



Mammalian Structure. — Having thus explained 

 the position of the mammals with respect to the 

 other classes of vertebrates, we may now occupy 

 our attention with their peculiar structure of body, 

 which in the most highly developed types of this 

 class has attained the highest grade capable of 

 being reached by the bodily structure of animals. 



General Characters. — In almost all mammals 

 the different parts of the body, such as the head, 

 neck, trunk, and its direct continuation the tail, are 

 strictly marked off from each other, and if these 

 demarcations are sometimes not very clearly seen 

 in the living animal, they are nevertheless always 

 plainly recognizable in the skeleton. The neck 

 may be shortened and so concealed by the flesh 

 that the head appears to be directly connected with 

 the trunk, as, for example, in the whales; but in 

 the skeleton one can always distinguish the neck 

 vertebra;, almost invariably seven in number what- 

 ever the length of the neck may be. This con- 

 stancy in the number of the neck vertebra;, a con- 

 stancy to which a few sloths and the manatee form 

 the sole exceptions, is all the more striking since 

 it is not found to hold good in the case of the other 



