AFFINITIES. 481 



to which the term uterus is applied. The embryo is provided 

 with an amnion and allantois and, excepting in the Monotremata 

 and most Marsupialia, is always connected with the uterine wall 

 by means of the allantois ; the combined structure formed 

 by the union of foetal and maternal tissues being called the 

 placenta. 



Such are the principal characters of the Mammalia. It will 

 be seen that the group is a very well defined one and clearly 

 marked off from the other vertebrate classes. As to its origin in 

 evolution we have nothing to say for the very good reason that 

 there are no facts by which we ca.n arrive at any conclusion on 

 the subject. It may however be noted here that, if we except 

 certain doubtful forms from the Eocene, the Monotremata are 

 not found fossil till the Pleistocene, and that the earliest known 

 fossil mammals appear, so far as can be judged from their some- 

 what scanty remains, to have been of a small and rather special- 

 ised type. It is true that these Triassic and Jurassic Mammalia 

 are classified with or in the neighbourhood of the Marsupialia, 

 but it cannot be asserted that this is done on any substantial 

 evidence. 



But we may say something with regard to the relations of 

 Mammalia to other classes of Vertebrata. Though a well defined 

 group they present rather close points of contact with living rep- 

 tiles through the Monotremata (p. 525) and especially with the 

 extinct Anomodontia (p. 398). The resemblances between 

 Monotremata and living reptiles are specially interesting be- 

 cause they concern the soft (urinogenital organs, ovum, etc.) as 

 well as the hard parts (shoulder girdle, etc.). Taking the totality 

 of these resemblances they reduce the important features of soft 

 parts which are peculiar to mammals to the red blood-corpuscles, 

 the aortic arch, the mammary glands and the hairs. Treating 

 the skeleton in the same way, we find that, having regard to the 

 Anomodontia and the Monotremata there is hardly a character 

 of any importance which can be said to be peculiar to Mam- 

 malia. We thus reach the conclusion that the gap between 

 reptiles and mammals is not a large one, that it is not indeed 

 larger than that between reptiles and birds, and that reptiles, 

 birds, and mammals constitute a natural group, more homo- 

 geneous than the group Ichthyopsida or even than the class 

 Pisces. Among the characters of the group so constituted w.> 



z. ii 1 1 



