spires of form

quotes from spires of form: glimpses of evolution by victor b scheffer

"sex ambivalence is rare among vertebrate animals, being known only in certain fishes, in one family of turtles, and in one species of alligator. it is not, however, uncommon among lower animals. for example, the larva of the marine echiurid worm, bonellia, is sexually indifferent. if it happens to settle in a population where females are abundant it becomes a male, and vice versa. 'thus,' writes evolutionist george williams, 'each individual adjusts its sex to the opportunities presented by its demographic environment.'"

"i wish to make clear that it is first the individual silverside or echiurid worm, and second its group, that benefits from the ability to mature either as male or female. "

"slugs and earthworms (for example), although not self-fertilizing, are equipped with both ovaries and testes. copulating individuals line up belly to belly with their heads pointing in opposite directions, to mutually discharge sperms into the other's body."

"so, unisexual reproduction is a strategy that quickens the reproductive rate of a species. breeding while still in the larval stage is another. paedogenesis (literally, 'descent through children') is practiced by the aquatic tadpoles of a mexican salamander. called locally an axolotl, each tadpole matures sexually, engages in courtship, and produces eggs or sperms before it reaches adulthood. however, an axolotl can be forced to metamorphose into a dry-land adult by treating it with thyroxin and by lowering the water level of its pond, thus making gill breathing more difficult and lung breathing easier.

when the axolotl was discovered it was thought to represent a new, strictly aquatic, gill-breathing race. later it was found capable of maturing into a land dwelling tiger salamander very like those that breed over much of north america. thus an 'axolotl' is simply an aberrant tiger salamander which, constrained by the poverty of its habitat, begins to reproduce as soon as it can, even before it has reached its potential adult size."

"in his study of crowding, calhoun looked also at the spatial distribution of wild, free-living mammals such as mice, shrews, and gophers, that typically defend individual territories. ideally, each territory would be six-sided, for the hexagon is the ideal unit in a tightly packed, two-dimensional configuration. (witness the honeycomb cell.) noting that an animal living in a field of hexagonal territories has six nearest and eighteen next-nearest neighbors (total twenty-four), calhoun suggested that the magic number six has left its imprint on man's society."

"limulus, the horseshoe crab, is the last of an ancient line. it is little changed from ancestors who swarmed in the triassic seas more than 300 million years ago. now ageless, suspended in time, it stands apart, neither a proper crustacean (among the crabs and their kin) nor a proper arachnid (among the spiders and their kin). . .  to reflect on the endurance of limulus is to wonder, does evolution move in one direction or does it occasionally reverse itself? does 'progress' describe its motion through time? because natural selection depends in part on opportunism, reverse evolution or devolution is theoretically possible."

"almost no animal organ performs quite the same function for which it was earlier adapted. the flippers of whales and the wings of bats, now used in swimming and flying, stem from the forepaws of terrestrial mammals, and still earlier from the forefins of fishes. parts of the gill-bearing skeleton of ancient fishes, now transformed and scarcely recognizable, are the bones and cartilages of the adam's apple you can feel at the base of your throat. and the three small bones in the human ear that carry sound from the eardrum to the auditory nerve have direct antecedents in reptilian jawbones."