32 Evolution and Distribution of Fishes 



and of Australia, that are either semi-lacustrine expanses, 

 or are subject to periodic floods annually or for short 

 periods, during which time considerable transfer of detritus 

 and rock-forming materials occur. The observations of 

 Baldwin Spencer, of Graham Kerr and others, along such 

 lines will be referred to later. 



But as depicting the possible geological changes effected 

 where the Australian dipnustean Neoceratodiis occurs, 

 Spencer's description {15; 81) deserves quotation. He 

 says, regarding the wide flood-plain of the Burnett river: 

 "In the rainy season, the creeks, dry in summer, become 

 converted into roaring torrents; the river rises suddenly, 

 as much sometimes as 50 feet in a very few days, and down 

 from the hills and the country round an enormous amount 

 of sand is swept suddenly into the water. When once the big 

 sandbanks of the river have been seen in dry weather, it 

 is easy to realize what a vast amount of sand must be swept 

 down into the stream at flood-time every year." 



While many of the larger rivers of the present day show 

 an equally marked rise and fall of the waters, some develop 

 flood plains of much greater extent than the above. Thus 

 the Amazon may spread in places to a width of 300-400 

 miles during the wet season, while it as well as the Congo, 

 Niger, Zambezi and many smaller rivers, may form exten- 

 sive lakes that vary from shallow marshy areas to sheets of 

 several feet in depth. These, as now existing, give us some 

 faint idea of the enormous geologic changes that must have 

 gone on at a greatly more rapid rate during the palaeozoic 

 period. 



As will appear later when the physical and biological 

 environment of fishes is traced out, it is highly probable 

 that the remains of many of these were stranded over such 

 flood-plains as the above. They may then have been covered 

 by a deposit of mud, or muddy sand, or of fine sand, and on 

 retreat of the waters to their regular bed, the deposit 

 might have been exposed to hot suns which would cake 

 together the whole of the enclosed fishes. Later on, and 

 within a few days at most, the entire mass would become 

 dried and preserved in comparatively natural positions. 



