首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
Comparatively few fresh-water species of fishes are limited in their distribution to a single river system, yet not many are fou
Comparatively few fresh-water species of fishes are limited in their distribution to a single river system, yet not many are fou
admin
2015-06-23
37
问题
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 fishes of the Mississippi Valley are generally different 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(分水岭), is exactly identical, such streams do have many species in common. 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, the bluegill sunfish, and the largemouth bass. None of these species can endure salt water, so they cannot move 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 pike, for example, is found in the Atlantic streams from Maryland northward, and the brook trout and yellow perch occur only in the streams from North Carolina southward.
How the present distribution came about must remain a matter of guess. 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 passageway for fishes. It is also possible that water birds may accidentally carry fish or spawn(卵)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. Then 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 United States has been found, and that is the eel, 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.
Efforts to deliberately transplant the bluegill sunfish and the largemouth bass into Panama would probably fail due to______.
选项
A、new environment
B、saltiness
C、temperature
D、rainfall
答案
C
解析
推理题。最后一段第二句指出限制鱼类分布扩大的主要原因是温度。这一原因成为美国淡水鱼向墨西哥、巴拿马等南美国家迁徙的主要障碍。
转载请注明原文地址:https://kaotiyun.com/show/9Jl7777K
0
大学英语四级
相关试题推荐
IsCollegeaWorthyInvestment?A)Whyarewespendingsomuchmoneyoncollege?Andwhyarewesounhappyaboutit?Weallseem
IsCollegeaWorthyInvestment?A)Whyarewespendingsomuchmoneyoncollege?Andwhyarewesounhappyaboutit?Weallseem
Manycollegestudentstodayownpersonalcomputersthatcostanywherefrom$1000toperhaps$5000ormore.【B1】______,itisn
AMessontheLadderofSuccessA)ThroughoutAmericanhistorytherehasalmostalwaysbeenatleastonecentraleconomicnarrati
Whenyouthinkaboutthegrowthofhumanpopulationoverthelastcenturyorso,itisalltooeasytoimagineitmerelyasani
Americanuniversitiesarerushingtowardsawirelessfuture.Theyareinstallingnetworksthatletstudentsandteacherssurfth
Averyimportantworldproblemistheincreasingnumberofpeoplewhoactuallyinhabitthisplanet.Thelimitedamountoflanda
IstheInternetMakingUsForgetful?A)AtouristtakesapictureoftheEmpireStateBuildingonhisiPhone,deletesit,thenta
IstheInternetMakingUsForgetful?A)AtouristtakesapictureoftheEmpireStateBuildingonhisiPhone,deletesit,thenta
IstheInternetMakingUsForgetful?A)AtouristtakesapictureoftheEmpireStateBuildingonhisiPhone,deletesit,thenta
随机试题
DNA解链温度是指
临床试验和生物等效性试验是新药的应根据药品的特性选用不易破损的包装,以保证药品在运输、贮藏、使用过程中的质量
判决执行前先行羁押的,羁押1日折抵刑期2日的刑种是:
某企业(增值税一般纳税人)2017年8月在生产领用原材料时,发现上月购进的不合税价为10000元、增值税额为1700元的原材料质量存在问题,已知其在购进时已支付货款并取得增值税专用发票,同时已经认证抵扣。现与销货方协商,将这批原材料全部退回销售方,收到销售
根据税收征收管理法律制度的规定,下列各项中,属于纳税担保范围的有()。
产品质量责任是一种综合的_________。
Athirstybeewenttoarivertodrink.Asitwasdrinking,thebeewas【C1】______awaybytherunningwater.Akindbirdsawthat
()是夜视器官,它们在昏暗的照明条件下起作用。
现有长方形木板340张,正方形木板160张(如下图左),这些木板恰好可以装配成若干个竖式和横式的无盖箱子(如下图).装配成的竖式和横式箱子的个数分别为().
No.10DowningStreetistheresidentof______.
最新回复
(
0
)