"There are many examples in evolution of a vital adaptation taking place at a cost. Anatomically, evolution seems to create a series of compromises; advantages tempered by being achieved against the constraints of the existing raw material. Upright walking has the cost of disorders in the back and acute difficulties in childbirth. Speech has the cost of a structure of the larynx which is extremely dangerous. The fact of these compromises does of course emphasize the power of the need that the advantage be achieved. If it is dangerous to speak, the pressure in its favor must have been very strong."