DISTRIBUTION OF THE FISHES 
GEOLOGICAL BASIS OF DISTRIBUTION 
In no part of the world do distributional studies present more interesting 
correlations between the hydrography and topography on the one hand, and the 
facts of plant and animal distribution on the other. Whether we accept or reject 
the Archhelenis theory to account for the beginnings of certain families of fresh- 
water fishes in South America, we still have ample evidence of the part played by 
the old land masses, Archiguyana and Archibrazil. 
(1) THe ORIGIN OF THE LAND Forms 
Throughout much of the Palaeozoic Era and the early Mesozoic, the portion of 
the South American continent with which we are concerned existed as an immense 
geosyncline, or trough, of greater or less extent below sea level. The older Paleo- 
zoic continents existed at that time farther eastward where the matured highlands 
of Brazil and the Guianas are today. Materials originating in the eastern high- 
lands probably contributed to the upbuilding of the immense depths of sedimentary 
formations in this western trough. The Brazilian and Guiana highlands were 
doubtless then of much greater height and more rugged form where now reduced 
to maturity. There is less evidence that an additional nearby continent existed 
on the Pacific side which might have added its contributions to the upbuilding of 
the many thousand feet of andean sedimentary rocks. At any rate the formations 
are there, derived from adjacent highlands, of enormous horizontal and vertical 
magnitude, requiring a correspondingly great total of submerged area of sea bottom, 
now a lofty mountain system. Either great deeps existed over a long period of 
time, or, more probably a long series of shorter periods of subsidence accompanied 
by aggradation were occurring and recurring throughout the period. This period 
of accumulation of sedimentary formations may be considered the first step. It is 
closely paralleled by events in the northern hemisphere, with the older Appalachian 
system contributing to a great syncline in the western ocean while weathering to 
maturity, and the new, rugged Rocky Mountain system rearing itself along the lines 
of the syncline, bearing aloft the sedimentary accumulations formed in it. 
In the middle Mesozoic a second step has begun. Diastrophic rearrangements 
of enormous magnitude were in order in many parts of the earth. In this area they 
continued intermittently throughout the Cretaceous, culminating in a series of 
folds of the old geosyncline forming mountain chains. This phase was completed 
by the end of that period, and the youthful Andes of the early Cenozoic were born, 
then of moderate elevation. (Schuchert and Dunbar, page 354.) 
The third stage appears to have been a period of comparative diastrophic 
inactivity through the earlier half of the Cenozoic, at least in much of the region, 
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