THE EFFECT OF MAN 551 



plants were to be found 145 miles away, but by 1918 this had receded 

 about 25 miles farther. The tendency of the grossly contaminated area 

 to creep downstream has been increased by the construction of re- 

 taining dykes to decrease the amount of land overflowed by the height- 

 ened level of the river. The lower river is less affected than it would 

 have been otherwise, owing to the construction of aeration dams in the 

 upper stream. 



Below the region of heaviest contamination the amount of plank- 

 ton increased greatly after the opening of the drainage canal. For 

 1897-1898 plankton organisms averaged 3 cc. per cu. m. of water 

 throughout the year; for 1909-1910 the average was 5.07 cc. The 

 amount present during April, May, and June, when the newly hatched 

 fish are plankton feeders, increased from a mean of 6.6 cc. to 17.9 cc. 

 Coincident with this increase in food organisms for young fishes and 

 with the increased size of breeding grounds made available by the 

 higher water level, the number of fish caught in the lower Illinois 

 River increased in the decade following the opening of the drainage 

 canal. The explanation of this increase is complicated by the fact that 

 European carp had become established in the river shortly before the 

 opening of the canal and found the new conditions in the lower river 

 favorable for development. It must not be forgotten that in achieving 

 this result the upper hundred of the 270 miles of the length of this 

 river had been rendered unproductive so far as commercial fishes were 

 concerned. Unfortunately this is not atypical in America; in the upper 

 Mississippi River, about 100 miles of flow is needed to allow the 

 stream to recover from the sewage dumped into the river by Minne- 

 apolis and St. Paul; 18 for the first half of this distance the river is 

 grossly polluted. 



Intentional and unintentional transport by man. — In Chapter V, 

 in discussing the means of animal dispersal, attention was called to 

 man's effectiveness in transporting his domestic animals which have 

 frequently become wild in their new environment, as have horses in 

 South America, cattle in the same continent and in Australia, pigs and 

 goats on many islands. The similar introduction of the dingo into 

 Australia by the aborigines, and later of rabbits there and elsewhere 

 by the Europeans, has excited much comment. Game animals have 

 also been distributed by this means; the fallow deer was introduced 

 into central Europe from the Mediterranean province and the pheasant 

 from the Pontic districts; the latter has since been released in numbers 

 in North America ; the mufflon has been introduced in the Carpathians 

 and the Harz in recent times; even kangaroos have been freed in 

 the game preserves of large landholders in England. 



