Life starts in the sea. There it attains to an extraordinary efficiency. The fishes give rise to types which are so successful (such for instance as the sharks) that they have lasted on unchanged until to-day. The path of ascending evolution did not however lie in this direction. In Evolution Dr. Inge’s aphorism is probably always right: ‘Nothing fails likes success.’ A creature which has become perfectly adapted to its environment, an animal whose whole capacity and vital force is concentrated and expended in succeeding here and now, has nothing left over with which to respond to any radical change. Age by age it becomes more perfectly economical in the way its entire resources meet exactly its current and customary opportunities. In the end it can do all that is necessary to survive without any conscious striving or unadapted movement. It can therefore beat all competitors in the special field but equally on the other hand should that field change it must become extinct. It is this success of efficiency which seems to account for the extinction of an enormous number of species. Climatic conditions altered. They had used up all their resources of vital energy in adapting to things as they were. Like unwise virgins they had no oil left over for further adaptations. They were committed, could not readjust and so they vanished.
Heard argues that evolutionary success can create its own vulnerability. A species may become so efficiently adapted to a particular environment that most of its capacities are committed to existing conditions. This specialisation gives it an advantage while those conditions remain stable, but leaves less flexibility when the environment changes. Heard uses long-lived marine species and the extinction of highly specialised forms to illustrate this tension between immediate efficiency and long-term adaptability.
The quotation then develops the biological argument into a broader philosophical point. Drawing on Dr Inge’s aphorism that ‘nothing fails like success’, Heard suggests that complete adjustment to present circumstances can reduce the capacity to respond to the unexpected. His image of the unwise virgins running out of oil presents adaptability as a reserve that must not be exhausted by present demands. The passage therefore contrasts optimisation for current conditions with resilience in the face of change.