76 THE GENESIS OF SPECIES. [CHAP. 



concurrence of some other and internal natural law or laws 

 co-operating with external influences and with "Natural 

 Selection" in the evolution of organic forms. 



It must never be forgotten that to admit any such con- 

 stant operation of any such unknown natural cause is to 

 deny the purely Darwinian theory, which relies upon the 

 survival of the fittest by means of minute fortuitous 

 indefinite variations. 



Amongst many other obligations which the author has 

 to acknowledge to Professor Huxley, are the pointing out 

 of this very difficulty, and the calling his attention to the 

 striking resemblance between certain teeth of the dog and 

 of the thylacine as one instance, and certain ornithic 

 peculiarities of pterodactyles as another. 



Mammals l are divisible into one great group, which 

 comprises the immense majority of kinds termed, from 

 their mode of reproduction, placental Mammals, and into 

 another very much smaller group, comprising the pouched- 

 beasts or marsupials (which are the kangeroos, bandicoots, 

 phalangers, &c., of Australia, arid the true opossums of 

 America), called implacental Mammals. Now the placental 

 mammals are subdivided into various orders, amongst 

 which are the flesh-eaters (Carnivora, i. e. cats, dogs, otters, 

 weasels, &c.), and the insect-eaters (Insectivora, i.e. moles, 

 hedgehogs, shrew-mice, &c.). The marsupial mammals also 

 present a variety of species (some of which are carnivorous, 

 whilst others are insectivorous), so varied in form that it 

 has been even proposed to group them into orders parallel 

 to the orders of placental beasts. 



1 I.e. warm-blooded animals which suckle their young, such as apes, 

 bats, hoofed beasts, lions, dogs, bears, weasels, rats, squirrels, armadillos, 

 sloths, whales, porpoises, kangaroos, opossums, &c. 



