706 MAMMALIA. 



ral view of the urinary system of Mammalia, to noticing in detail 

 those varieties that occur in the disposition of the bladder and ure- 

 thra of some of the lower tribes, in conformity with the different 

 types of organization presented by their sexual organs ; these, how- 

 ever, must not be lost sight of in following out the developement of 

 the reproductive apparatus, from the oviparous races to the most 

 perfect and highly gifted members of the animal creation. It is to 

 this important subject that we must now invite the attention of the 

 reader. 



(833.) The oviparous Vertebrata lay eggs, and their young are 

 perfected without further nourishment derived from the maternal 

 system than is contained within the egg itself. In our own species, 

 and throughout all the races of Mammalia found on the European 

 continent, the females produce their young alive and fully formed, 

 capable of independent existence, but, nevertheless, nourished for 

 a considerable period by milk derived from the breast of the 

 mother. The distinction, therefore, between an oviparous and a 

 viviparous creature would appear to be sufficiently broad, and the 

 physiological relations between them as remote as possible. 



The student, however, who has followed us thus far through 

 the long series of living beings that have successively presented 

 themselves to our notice, must naturally expect that between ani- 

 mals so dissimilar in their economy as the Bird and the Mammal, 

 intermediate types of organization must occur, and that the trans- 

 ition from one to the other is here, as elsewhere, gradually ac- 

 complished. 



In this respect his expectations will be by no means disap- 

 pointed. The Ornithorynchus paradoxus and the Echidna, ani- 

 mals met with only in the continent of New Holland, are most 

 obviously connecting links between these two grand classes ; and 

 it is, therefore, with the history of these strange animals that we 

 must commence our examination of the Mammiferous generative 

 system. 



The Ornithorynchus paradoxus well deserves the specific epi- 

 thet applied to it by zoologists. It has, indeed, the form of a 

 quadruped, and its body is covered with hair, and not with feathers; 

 but its mouth is the beak of a duck, and upon its hind feet, which 

 are broadly webbed, the male carries a spur not unlike that of a 

 barn-door fowl. Having the beak of a bird, how is the creature 

 to suck ? Nevertheless the females have mammary glands well 

 developed, but destitute of prominent nipples, so that the mode in 



