How completely this applies to man, the latest product of organic evolution. The brain is that part or organ in him which has been developed "in an extraordinary degree, in comparison with the same part" in other Primates, and which has become _highly variable_. Whatever may have been the favouring causes which secured his immediate progenitors such modification of brain as advanced him in intelligence over "allied species," the fact abides that in this lies the explanation of their after-history; the arrest of the one, the unlimited progress of the other. Increasing intelligence at work through vast periods of time originated and developed those social conditions which alone made possible that progress which, in its most advanced degree, but a small proportion of the race has reached. For in this question of mental differences the contrast is not between man and ape, but between man savage and civilized; between the incapacity of the one to count beyond his fingers, and the capacity of the other to calculate an eclipse of the sun or a transit of Venus. It would therefore seem that Mr. Wallace should introduce his "spiritual essence, or nature," in the intermediate, and not in the initial stage. As answer to Mr. Wallace's argument that in their large and well-developed brains, savages "possess an organ quite disproportioned to their requirements," Huxley cites Wallace's own remarks in his paper on Instinct in Man and Animals as to the considerable demands made by the needs of the lower races on their observing faculties which call into play no mean exercise of brain function.