794 THE AMERICAN JOURNAL OF SOCIOLOGY
Dynamic, also, are such operations as modify the physical envi- ronment. In explaining the varying destinies of a people, says Metchnikoff,
one can neglect the slow geological and climatic changes ; on the other hand, the modifications that human industry, the accumulated labor of suc- cessive generations, produce in the nature of the country have a very great
importance Thus the prehistoric settlers in the Nile valley handed
over to their descendants of the Memphite epoch an environment very differ- ent from the one they had received from the hands of nature Later,
important works, such as the reservoir of Fayoum, modify considerably the physical conditions confronting the Egyptians of the Theban period.
Dykes, levees, canals, drains, causeways, and roads alter the economic plane on which society rests. In China and about the Mediterranean deforestation has produced momentous changes. Mining, clearing, "breaking," reclaiming, inclosing, improving, as well as the destruction of pests, have a dynamic effect, seeing they lessen the material they have to work upon. The digging of the precious metals renders them in time so plentiful that the money economy supplants the natural economy and society is profoundly transformed. The casual acclimatization of alien economic plants and animals in a region may prevent social standstill.
Certain modifications of the human breed come about as accumulated incidental effects. As the ax devours the forest and the plow the prairie, the hunting and nomad types starve and man is tamed. Trade in time eliminates the impulsive type and fills the earth with calculators. With the lapse of genera- tions, an institution like monasticism or sacerdotal celibacy by its unnoticed selective working alters the bench-mark of race- fiber to which all social structures conform. A bloody penal system, besides intimidating the evil-disposed, incidentally extirpates the criminal type, and so paves the way for a milder code. Monogeny, child-marriage, primogeniture, indiscriminate almsgiving, religious persecution, and militarism all accumulate unsuspected but far-reaching results.
History furnishes striking instances of large changes brought about by processes which left behind them a little more or less of something. The destruction of the middle class, the curiales, in
