"Evolution is commonly conceived to imply in every thing an intrinsic tendency to become something higher, but this is an erroneous conception of it. In all cases it is determined by the cooperation of inner and outer factors.... Usually neither advance nor recession results, and often, certain previously-acquired structures being rendered superfluous, there results a simpler form. Only now and then does the environing change initiate in the organism a new complication, and so produce a somewhat higher type. Hence the truth that while for immeasurable periods some types have neither advanced nor receded, and while in other types there has been further evolution, there are many types in which retrogression has happened.... Of all existing species of animals, if we include parasites, the greater number have retrograded from a structure to which their remote ancestors had once advanced. Often, indeed, progression in some types involves retrogression in others. For always the more evolved type, conquering by the aid of its acquired superiority, tends to drive competing types into inferior habitats, and less profitable modes of life; usually implying some disuse and decay of their higher powers.

"As with organic evolution, so with super-organic evolution. Though, taking the entire assemblage of societies, evolution may be held inevitable as an ultimate effect of the coöperating factors, intrinsic and extrinsic, acting on them all through indefinite periods of time, yet it cannot be held inevitable in each particular society, or even probable. A social organism, like an individual organism, undergoes modifications until it comes into equilibrium with environing conditions, and thereupon continues without further change of structure. When the conditions are changed, meteorologically or geologically, or by alterations in the Flora and Fauna, or by migration consequent on pressure of population, or by flight before usurping races, some change of social structure is entailed. But this change does not necessarily imply advance. Often it is toward neither a higher nor a lower structure. Where the habitat entails modes of life that are inferior, some degradation results. Only occasionally is the new combination of factors such as to cause a change constituting a step in social evolution, and initiating a social type which spreads and supplants inferior social types. For with these super-organic aggregates, as with the organic aggregates, progression in some produces retrogression in others; the more-evolved societies drive the less-evolved societies into unfavorable habitats, and so entail on them decrease of size, or decay of structure.

"Direct evidence forces this conclusion upon us. Lapse from higher civilization to lower civilization, made familiar during school-days, is further exemplified as our knowledge widens. Egyptians, Babylonians, Assyrians, Phoenicians, Persians, Jews, Greeks, Romans—it needs but to name these to be reminded that many large and highly-evolved societies have either disappeared, or have dwindled to barbarous hordes, or have been long passing through slow decay. Ruins show us that in Java there existed, in the past, a more developed society than exists now; and the like is shown by ruins in Cambodia. Peru and Mexico were once the seats of societies large and elaborately organized, that have been disorganized by conquest; and where the cities of Central America once contained great populations,