246 



EXPLANATIONS. 



suggested that the peculiar organization of the marsu- 

 pials points to their having been derived through a dif- 

 ferent medium from other mammals. The critic, eagei 

 to let nothing escape, tells us that there are other land 

 mammal3 lower in organic type than the marsupials. 

 One answer to this objection might be found in an expla- 

 nation of my views respecting the ornithic descent of 

 these animals ; but I am unwilling to pause upon such 

 an inferior matter, and will therefore meet him with the 

 question, if any other mammals show that lowly grade ot 

 organization which is marked by the absence of a pla- 

 centa ? " There are no other organic types," he says, 

 ** to which they [the marsupials] offer the shadow of a 

 near affinity. They are therefore in direct antagonism 

 with the scheme of regular development." To this it 

 may be replied, that the affinity of the marsupials to the 

 oviparous vertebrata is admitted by every naturalist, 

 being shown in the small size of the brain and conse- 

 quent exposure of the cerebellum, the absence of the 

 septum lucidum and corpus callosum in the brain, and 

 various other traits. Professor Rymer Jones, of King's 

 College, whose testimony on such a point will be ad- 

 mitted by the reviewer, speaks of the marsupials as 

 ** connecting links between the oviparous and placental 

 vertebrata." Striking traits of their affinity to birds are 

 shown, he says, in the structure of the ear and of the re- 

 productive organs.* In reality, the whole figure of the 

 cursorial bird, the small head upon the long neck, the 

 extreme length of the hinder limbs, and the imperfect 

 development of the fore extremities, as well as the ten- 

 dency of the feathers to a hair-like character, speak irre- 

 sistibly for its approach to certain marsupials. The orni- 

 thorhynchus is as clearly an advance from the natatorial 

 bird towards the rodent form, the latter being an order 

 whose osteological structure is allowed by every natural- 

 ist to be bird-like. New and curious illustrations of the 

 connection between the birds and the implacental mam- 

 malia are constantly appearing. We lately heard of a 

 bird which has a pouch for its young like the kangaroo,! 

 and Mayer has discovered in the female emeu a purse 

 form of certain organs, indicating an approach to the 

 marsupial in that part of structure which is the most dis 



* General View of the -Structure of the Animal Kingdom. 

 \ Magazine of Natural Plistory. 



