by John A. Davison
Evolution is thus seen as a series of blind alleys. Some are extremely short - those leading to new genera and species that either remain stable or become extinct. Others are longer - the lines of adaptive radiation within a group such as a class or subclass, which run for tens of millions of years before coming up against their terminal blank wall. Others are still longer - the lines that have in the past led to the development of the major phyla and their highest representatives; their course is to be reckoned not in tens but in hundreds of millions years. But all in the long run have terminated blindly. That of the echinoderms, for instance, reached its climax before the end of the Mesozoic. For the arthropods, represented by their highest group, the insects, the full stop seems to have come in the early Cenozoic: even the ants and bees have made no advance since the Oligocene. For the birds, the Miocene marked the end; for the mammals, the Pliocene.
And a few zoologists are beginning to recognize that evolution is slowing down, if not quite stopped. In a letter I had from Professor Julian Huxley only a few months ago he says, ‘I have often thought about your idea of the fading out of evolutionary potency, and though I cannot pretend to agree with some of the philosophical corollaries which you draw from it, I more and more believe that it is of great importance as a fact.’ (Broom, 1933).
A small minority of biologists, such as Broom (1933), still feel impelled to invoke ‘spiritual agencies’ to account for progressive evolution, but their number is decreasing as the implications of modern selection theories are grasped.
The reference to “spiritual agencies” by Broom was his suggestion that there had been a Plan, a word he capitalized.
Without referring to either Huxley or Broom, Pierre Grasse reached the same conclusions:
Facts are facts; no new broad organizational plan has appeared for severaland:
hundred million years, and for an equally long period of time numerous
species, animal as well as plant, have ceased evolving… At best, present
evolutionary phenomena are simply slight changes of genotypes within
populations, or substitution of an allele with a new one. (Grasse, The
Evolution of Living Organisms,1977 page 84.)
The period of great fecundity is over; present evolution appears as a
weakened process, declining or near its end. Aren’t we witnessing the
remains of an immense phenomenon close to extinction? Aren’t the small
variations which are being recorded everywhere the tail end, the last
oscillations of the evolutionary movement? Aren’t our plants, our animals,
lacking some mechanisms which were present in the early flora and
fauna? (Ibid, page 71).
Huxley, J. (1942) “Evolution: The Modern Synthesis.” Harper, New York and London.