224 THE TICTORIAN NATURALIST. [Vol. XXIII. 



America, and nowhere else. There are no giraffes in 

 Australia, and so on. There is a reason for these facts, 

 and one that can be found if we search diligently for it. 



It_is now accepted by zoologists that all animals now 

 existing have descended from pre-existing forms, and that 

 the diversity of animal life is due to modification during- 

 this descent. It is held that all lions, tigers, and other 

 cats, both large and small, have descended from common 

 ancestors. Similarly all mosquitoes spring from the same 

 ancestral mosquito-stock, and so on throughout both 

 animal and plant world. This, it may be noted in pass- 

 ing, is not the Darwinian theory. What Darwin did was 

 to offer an explanation of how the variations from the 

 original stock were emphasized and perpetuated, giving 

 rise to species and to the various^ larger groups. 



The mammals of Australia comprise marsupials and 

 egg-laying mammals, and a few higher forms which are 

 late-comers ; they are interlopers or strays from elsewhere. 



In the lecture the characters, of the platvpus and 

 echidna, or porcupine ant-eater, which are egg-laying 

 mammals, were briefly indicated, and by the aid of a 

 series of lantern slides, many of which were due to Mr. 

 Dudley Le Souef, the main kinds of marsupials were de 

 scribed. Running through the whole class of marsupials 

 is the clearly-seen bond of kinship. They are sprung all 

 from the same ancestral stock. 



The only other place in the world where marsupials 

 are found is America, and they are commoner and more 

 varied in South America. But the American marsupials, 

 as a whole, are not very closely allied to our Australian 

 ones, so that though both sets must have a common mar- 

 supial ancestry, yet it must have been very long ago that 

 they diverged. The two branches have had time to become 

 very different, and the time cannot have been short, for 

 the differences are great. 



Land mammals cannot cross the ocean ; even a narrow 

 strait is a barrier that sets definite bounds to their spread. 

 If, then, we find the same animals on an island as we do 

 on the neighbouring mainland, we must conclude that the 

 island was once part of the mainland, and that the 

 animals, passed over dry-shod. A very large proportion 

 of animals of all kinds, marsupials, egg-laying mammals, 

 birds, reptiles, frogs, freshwater fish, insects, worms, and 

 so on, are identical in Tasmania and Southern Victoria - 

 Indeed, the zoological relationship of Tasmania and 

 Southern Victoria are closer than those of Southern and 

 Northern Victoria. 



