Hen’s Teeth and Horse’s Toes by Stephen Jay Gould

at least encourage, intellectual modes different from our own. These smaller brains need not be viewed as direct adaptations to any prevailing condition. They represent, rather, inherited structural constraints that limit the range of specific adaptations fashioned within their orbit. The sailor’s style is a constraint that permits boobies to reduce their broods by exploiting a behavioral repertoire based on inflexible rules and simple triggers. Such a system would not work in humans, for parents will not cease to recognize their babies after a small and simple change in location. In human societies that practice infanticide (for ecological reasons often quite similar to those inducing siblicide in boobies), explicit social rules or venerated religious traditions—rather than mere duplicity by removal—must force or persuade parental action.
    Birds may have originally developed their brain, with its characteristic size, as an adaptation to life in an ancestral lineage more than 200 million years ago; the sailor’s style of intelligence may be a nonadaptive consequence of this inherited design. Yet this style has set the boundaries of behavior ever since. Each individual behavior may be a lovely adaptation, but it must be fashioned within a prevailing constraint. Which is more important: the beauty of the adaptation or the constraint that limits it to a permissible path? We cannot and need not choose, for both factors define an essential tension that regulates all evolution.
    The sources of organic form and behavior are manifold and include at least three primary categories. We have just discussed two: immediate adaptations fashioned by natural selection (exploitation by older booby siblings of their parents’ intellectual style, leading to easy dispatch of nest mates); and potentially nonadaptive consequences of basic structural designs acting as constraints upon the pathways of adaptation (the intellectual style of yes-no decisions based on simple triggers).
    In a third category, we find definite ancestral adaptations now used by descendants in different ways. Nelson has shown, for example, that boobies reinforce the pair-bond between male and female through a complex series of highly ritualized behaviors that include gathering objects and presenting them to mates. In boobies that lay their eggs upon the ground, these behaviors are clearly relics of actions that once served to gather material for ancestral nests—for some of the detailed motions that still build nests in related species are followed, while others have been lost. The egg-laying areas of masked boobies are strewn with appropriate bits of twigs and other nesting materials that adults gather for their mutual displays and then must sweep out of the guano ring to lie unused upon the ground. I have emphasized these curious changes in function in several other essays (see 4 and 11) for they are the primary proof of evolution—forms and actions that only make sense in the light of a previous, inherited history.
    When I wonder how three such disparate sources can lead to the harmonious structures that organisms embody, I temper my amazement by remembering the history of languages. Consider the amalgam that English represents—vestiges, borrowings, fusions. Yet poets continue to create things of beauty. Historical pathways and current uses are different aspects of a common subject. The pathways are intricate beyond all imagining, but only the hearty travelers remain with us.

4 | Quick Lives and Quirky Changes
    POSTHUMOUS TRIUMPH IS HOLLOW , however abstractly rewarding. Nanki-Poo refused Ko-Ko’s inducement to undergo a ceremonious public beheading rather than a private suicide: “There’ll be a procession—bands—dead march—bells tolling…then, when it’s all over, general rejoicings, and a display of fireworks in the evening. You won’t see them, but they’ll be there all the same.” And I never could figure out why America’s premier