11．Comparatively few fresh-water species of fishes are limited in their distribution to a single river system，yet not many are found on both sides of a high mountain ridge，such as the Rocky Mountains in North America．That is to say，the fish of the Mississippi Valley are ge nerally different and distinct from those of the Pacific slope．
While it is a well-known fact that the fish life in no two river systems，even though they empty into the sea on the same side of a divide，such streams do have many species in c ommon．]The principal rivers of the Atlantic slope of the United States，for example，contain several species common to all of them，including the bullhead catfish（鯰魚），the bluegill sunfish and the largemouth bass（鱸魚）．None of these species can endure salt water，so that they cannot now migrate from one river system to another．On the other hand，the more northern streams contain species not found in the southern ones，and vice versa．The common lake，for example，is found in the Atlantic streams from Maryland northward，and the brook trout（蹲魚）and yellow perch（鱸） emerge only in the streams from North Carolina southward．
How the present distribution came about must remain a matter of conjecture．It is quite probable that some of the streams，including those on opposite sides of a divide，may have been connected at one time．Again，streams may be entirely separate during normal weather，but an exceptionally heavy rainfall or the sudden melting of snow in the uplands sometimes causes floods which may form a temporary connection between them，providing a passage way for fishes．]It is possible，also that water birds may accidentally carry fish or spawn from one stream to another，or that man may be instrumental in such a transfer．
Evidently，then，fresh-water fishes may become distributed far beyond the confines of the stream of their origin．The chief factor in limiting the still wider distribution of species is temperature．This forms such an efficient barrier that comparatively few species of fresh-water fishes of the United States extend their range into Mexico．In Panama only one fish common to the fresh waters of the States has been found，and that is the eel（鰻），which is not strictly a fresh-water form，as it enters salt water to spawn and is taken in fairly salty water at other times．
32．According to the passage，B．
A．The Mississippi River and the Pacific slope are on the same of the Rocky Mountains．
B．The Mississippi River and the Pacific slope are on the two sides of the Rocky Mountains．
C．The Mississippi River and the Rocky Mountains are on the two sides of the Pacific slope．
D．The Pacific slope and the Rocky Mountains are on the two sides of the Mississippi．
33．In different river systems on one side of a divide，we may findA．
A．fishes of the same species．
B．fishes of the different origin．
C．a few rare fish species．
D．fishes that have the same life．
34．Species，which can't tolerate saltiness，may spread in to another river by the following mechanisms exceptD．
A．being carried by birds
B．flooded connections
C．humanity
D．spawning in the river mouth
35．Efforts to deliberately transplant the bluegill sunfish and the large-mouth bass into Panama would probably fail due toD．
A．eels B．salinity C．rainfall D．temperature．
分析 本文主要講述密西西比河谷和太平洋斜坡魚種區(qū)別的原因．
解答 BADD
32 B 細節(jié)題． 根據(jù)文章第一 段 yet not many are found on both sides of a high mountain ridge，such as the Rocky Mountains in North America．That is to say，the fish of the Mississippi Valley are ge nerally different and distinct from those of the Pacific slope．可知密西西比河谷和太平洋斜坡位于北美落基山脈的兩側，故選B．
33 A 細節(jié)題． 根據(jù)文章第一 段That is to say，the fish of the Mississippi Valley are ge nerally different and distinct from those of the Pacific slope． 可知在不同的河流系統(tǒng)劃分的一側魚的品種是一樣的，故選A．
34 D 細節(jié)題． 根據(jù)文章第三 段 可知不能忍受鹽的物種或許會通過鳥類，人類以及洪水的原因而進入另一條河流，故選D．
35 D 細節(jié)題． 根據(jù)文章最后一段The chief factor in limiting the still wider distribution of species is temperature． 可知限制物種廣泛分布的主要因素是溫度，故選D．
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