36 



THEORETICAL EXPLANATIONS OF THE DISTRIBU- 

 TION OF SOUTHERN FAUNAS. 



By Captain F. W. Huttox, F.R.S., Hon. Memb. L.S.N.S.W. 



On considering the present geographical distribution of land 

 and purely fresh-water vertebrates the first and most obvious 

 generalisation is that while the same or closely allied species are 

 widely spread in the northern hemisphere — through Europe, Asia, 

 and N. America — there is, in the southern hemisphere, a great 

 difference between those inhabiting S. Africa, Australasia, and 

 S. America. When we turn our attention to the marine verte- 

 brates — including the migratory fishes which pass a pa.rt of the 

 year in fresh water — we notice that the opposite is the case; for 

 while closely related species are widely difi'used in the southern 

 hemisphere, the seals, whales, sea-birds and fishes of the N. 

 Pacific differ considerably from those of the N. Atlantic. The 

 reason for these peculiarities is, of course, the peculiar configura- 

 tion of the land and sea, giving most of the land to the northern 

 and most of the sea to the southern hemisphere; and a necessary 

 conclusion is that the present configuration of the oceans and 

 continents must have remained much as it is now for a very long 

 time. Indeed oceans and continents could not have been widely 

 different from what they now are ever since most of the present 

 genera — and in some cases even families — of birds and mammals 

 came into existence; for, if such had not been the case, we should 

 not now find these genera and families isolated from each other 

 by barriers of land in the northern, and of sea in the southern 

 hemisphere. We may, therefore, safely infer that the physical 

 geography of the earth has not altered greatly during the latter 

 half of the Tertiary era. 



But when we pass from the general aspect of the question to 

 study the details, we find many exceptions (especially in the 

 distribution of the land plants and land animals of the southern 



