Birds that are superb underwater swimmers, such as penguins, or highly capable runners, such as ostriches, tend to lose their ability to fly. The engineering specifications for swimming or running conflict with those for flying. Most species, faced with such alternatives, are forced by selection into one adaptation or the other. Beings that hold all their options open tend to be eased off the world stage. Overgeneralization is an evolutionary mistake. But organisms that are too narrowly specialized, that perform exceedingly well but only in a single, restrictive environmental niche, also tend to become extinct… Nature has posed life a dilemma: to strike the optimum balance between the short-term and the long, to find some middle road between overspecialisation and overgeneralization.

~ Carl Sagan (1934-11-09 1996-12-20 age:62)