LXXXVI PRESIDENTIAL ADDRESS. 



and echidna, are only,, as it were, just emerging from reptility — 

 they still lay eggs, but they also suckle their young, though by 

 means of modified sudorific, and not sebaceous, glands, as in all 

 other mammals. In them, the pouch is only nascent. Next come 

 the marsupials, amongst whom the lacteal gestation, associated 

 with the pouch, is markedly emphasized, almost to the exclusion 

 of the placental, though, as shown by Hill in his brilliant investi- 

 gations, there are in certain marsupials indications of this, which 

 is the most characteristic feature of the Eutheria, in some of 

 which, again, we find traces of the pouch. 



The early ancestors of mammals must evidently have possessed 

 in their germ cells factors capable of determining, amongst other 

 things, the manner of feeding and carrying the young — not one 

 set of factors in one group and different ones in another, but, as 

 it were, a mixture of factors in all, some of which in different 

 groups have become dominant, while others have become over- 

 ridden, or inhibited. Otherwise, it seems difficult to understand 

 why in marsupials we have one fonn of gestation strongly and the 

 other only partly developed, and yet both present, and why in 

 Eutheria we have the strong development of the second 

 (placental), and yet traces of a pouch, indicating the possibility 

 of a prolonged lacteal gestation. Further still, in regard to the 

 possession of common factors such as these, which may well lead 

 to polygeneeis, we have the striking fact that within the limits 

 of the Marsupialia there are, while still retaining their marsupial 

 character, families each of which, so far, for example, as its 

 dentition is concerned, is closely similar to one or other of the 

 Eutherian orders, and adapted to the same method of life. We 

 find carnivorous forms, such as the Thylacine, whose skull and 

 dentition is built up on the same lines as the dog; Dasyurus and 

 Sarcophilus closely similar to the cat tribe; bandicoots and 

 Myrmecobius with distinct insectivorous dentition; kangaroos and 

 others with herbivorous dentition. To mention only one other, but 

 a most suggestive example, we have in the gnawing wombat the 

 incisors built on fundamentally the same plan as those of the 

 Rodent! a, with a coating of hard enamel almost, or entirely, con- 

 fined to the front surface. In fact, with the exception of the true 

 flying bats and sundry sea forms, we can parallel in the Mar- 

 supialia each distinctive group of the Eutheria, and yet it is 

 certain that the former have been isolated for long ages in A,us- 

 tralia, and, springing from common ancestors, have developed 

 along lines similar to those of the Eutheria. This suggests once 

 more that the early, but little, dift"erentiated, ancestors of mammals 

 must have possessed in their germ cells certain factors capable of 

 determining the development of their descendants along various 

 lines. To put it crudely, in some cases carnivorous, in others the 



