of physical structures profoundly modified in consequence, of the acquirement by generation after generation, through innumerable generations, of certain mental traits; we should have an example of physical structures profoundly modified, because the traits acquired caused the survival of individuals different from those which would otherwise have survived. In that case the acquired habit of slave-making, transmitted from preceding generations to succeeding generations, as men transmit language or property, has been the cause of the survival of those ant communities in which the individuals were most fit mentally and structurally for slave-making, for fighting, rather than of those communities in which the individuals were most fit for maintaining existence by their own industry, and therefore the cause of the evolution of the enormous jaws and their co-ordinated structures, and the concomitant retrogression of many other structures.
In man occur many examples of structural evolution and retrogression traceable to the persistent acquirement by generation after generation through many generations of various traits. His hearing and notably his sense of smell have retrogressed, because, owing to his growing powers of acquiring reason, these senses less and less influenced the survival rate. His teeth, and all the structures co-ordinated with them (jaw-bones, muscles, nerves, blood-vessels, &c.), have retrogressed, as has also his whole digestive apparatus, owing to his acquired habit of cooking his food; the survival rate being here beneficially influenced by an acquired trait, which rendered him above all other animals omnivorous, which enabled him to use for food a greater number of things than any other animal; and therefore, though man is no longer able to digest the raw food on which his remote ancestors subsisted, yet his acquired habit brings him into completer harmony with the environ-
