in Thoreau’s Journal:

The Roman writers Columella and Palladius warn not to build in a low valley or by a marsh, and the same rule is observed here to-day. In the West the prudent settler avoids the banks of rivers, choosing high and open land. It suggests that man is not completely at one with Nature, or that she is not yet fitted to be his abode. Adam soon found that he must give a marsh a wide berth, —that he must not put his bower in or near a swamp in the new country,  —else  he would get the fever and ague or an intermittent fever. 

Either nature may be changed or man. Some animals, as frogs and musquash, are fitted to live in the marsh. Only a portion of the earth is habitable by man. Is the earth improving or deteriorating in this respect? Does it require to be improved by the hands of man, or is man to live more naturally and so more safely?