ADAPTATION 199 



from Mexico to Patagonia, provided it may oc-^ 

 eupy the same sort of enviromnent in each 

 stream. There is more environmental differ- 

 ence in the different parts of a cross-section of a 

 river in the Amazon region, or in a mile of the 

 length of a small brook, than there is in the 

 pelagic region of streams from Mexico to Pata- 

 gonia. 



2. Geographical convergence. Each river is 

 made up of many different units of environment. 

 The pelagic area is but one of these. Muddy 

 bottom, weedy bottom, stagnant water, swiftly 

 flowing water, are other units. Each has its 

 pecuharly adapted fauna. Different members 

 of the same family may belong to different eco- 

 logical series, and different ecological groups are 

 made up of members of different families. In 

 shallow, swift water over gravel, in a small 

 stream, the adaptations required are a heavy 

 body, strong pectorals and ventrals, on which the 

 fish sits and which are held in readiness for sud- 

 den springs. The conditions and adaptations 

 are the same whether the stream be in North 

 America, in Cuba, or in South America. Fishes 

 are adapted to the conditions in each locahty, but 

 the adapted faunas in the three areas are not re- 

 lated. In North America, darters, or diminu- 

 tive perches, are adapted to this niche; in Cuba 

 it is members of the marine Gobies, and in South 

 America members of the versatile Characins and 

 catfishes. Shape and many other things count 



