32 MAMMALIAN DESCENT. [Lect. II. 



The structure and development of this type is both 

 Eeptilian and Avian. Yet this is but a clumsy way of 

 expressing it. The three groups — Reptiles, Birds, and 

 low (Prototherian) Mammals— correspond in many im- 

 portant points, so much so, as to suggest a common root 

 for all these three branches. In some things which are 

 common to aU three, in the number and relation of parts, 

 these low mammals are more archaic than the exist- 

 ing reptiles, and very much more than the existing 

 birds — not excluding the flat-breasted Ostrich and Emeu. 

 But in the higher, winged birds, the parts that are 

 distinct and simple in the Ornithorhyiichus and Lizard, 

 are found to be confluent and compound, and to undergo 

 a practical metamorphosis into exquisite new structures 

 for new functions. 



The primitiveness of this low mammal is weU seen 

 in its shoulder-girdle ; its skull, as yet only partially 

 worked out by me, shows characters of the same sort, 

 much more remarkably. The name given to this low 

 order — Monotremes — suggests in one word, that which 

 is most striking in these t3rpes, namely, that their renal 

 and reproductive organs are constructed in a similar 

 manner to those of a reptile or bird. In this respect, 

 even the common Mole is a high and noble creature in 

 comparison with the Ornithorhynchus, or the Echidna. 



The limbs of the Monotremes are normally pentadac- 

 tyle (or five-toed), but excessively specialised, in each 

 case, in relation to the habits of the creature. The hip- 



