23.3 THE ZOOLOGIST. 



six in the Australian Seas only. The number of cosmopolitan 

 genera is about twelve : 90 per cent, of the genera are limited to 

 small districts. 



The Australian Seas are the richest in peculiar genera and 

 families, principall}' belonging to those grouj)s -which we consider 

 as the most highly developed. Of the Calcispongice, the Teichonidce 

 and SylleihidcB are confined to Australia, and the Leuconiclce are 

 very plentiful. In the same way the highest Chondros2)ongi(e, the 

 Tethydce, are represented in the Australian Seas by seven genera, 

 whilst from all other parts of the globe only three genera are 

 known. The number of species of Australian Chondrosidcs and 

 Chondrillidce is likewise greater than from other parts. The 

 Keratosa, the most highly developed Cornacuspongice, are repre- 

 sented by thirty-three genera, with more than two hundred 

 species, in the Australian Seas ; whilst only nine genera, with 

 about fifty species, are known from other parts of the globe. 

 The lower forms of Cornacuspojigm are distributed pretty equally. 



The Ectyonmce and Chalinince, which are rich in spongin, are 

 prevalent in the Australian Seas, whilst the Espierellin(e, which 

 possess very little spongin, are rare. The latter, as well as 

 the ChoristidcB and Suheritidm, are most abundant in the North 

 Atlantic Ocean. Australia, the land-fauna of which appears an 

 age behind that of other continents, harbours on its shores the 

 most highly developed sponge-fauna. 



The localisation of the different groups of marine sponges 

 appears particularly remarkable, considering that the fresh- 

 water sjjonges are more or less cosmopolitan. The familj^ Spon- 

 gillidcB is represented in all quarters of the globe, and the common 

 English fresh-water sponge, or very insignificant varieties of it, 

 occurs everywhere. This is particularly remarkable in the case of 

 Australia. Whilst the marine sponges on its coast are entirely 

 different from those of the North Atlantic, the fresh-water sponges 

 found in the isolated rivers of Australia are the same or very 

 similar to those of our English streams, although the physical 

 conditions are as different as they can be. This shows that the 

 continued inbreeding of the fresh-water sponges caused by their 

 isolation destroys their variability, whilst the continued hybridi- 

 sation of the marine sponges causes a continual renewal of their 

 variability, and further demonstrates that the efficient cause of the 

 variability of species must be sought in their hybridisation. 



