Is Evolution Finished?

Is Evolution Finished? - T. A. Goudge As all the living forms 0/ life are the lineal descendants oj those which lived long before the Cambrian epoch, we may feel cerlain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. Hence we may look with some confidence to a secure future oj great length .. . to progress towards perfection. CHARLES DARWIN In the first flush of enthusiasm produced by the work of Charles Darwin, his achievement was often compared with that of Newton. Just as Principia Mathematica formulated laws which govern the domain of inanimate matter, so, it was said, The Origin of Species formulated laws which govern the domain of life, at least in its historical aspect. Furthermore, just as physical bodies must obey the law of universal gravitation, so living bodies must obey the law of evolution by natural selection. It was an easy step from this view to the belief that the evolutionary process described by Darwin is not just something which happened in the past. The process is also going on in the present, and is destined to continue throughout "a secure future of great length." For most Darwinians in the nineteenth century, biological evolution was literally a work in progress. Today this optimistic belief is far from being unchallenged. No informed person can doubt that there has occurred on the earth during the past two thousand million years an evolutionary process which has brought about the enormous diversity and complexity of organisms from remote ancestors who were relatively simple and homogeneous in structure, function and type. But some informed persons doubt whether this process is now going on. There are reasons for believing, they argue, that evolution is virtually, if not actually, finished. In the present paper I propose to outline certain of their arguments and make a brief estimate of their cogency. Since man, like all living things, must be subject to the evolutionary forces which produced him, he can scarcely be uninterested IS EVOLUTION FINISHED? 431 in the question whether they are or are not still at work, especially if he surveys his prospects in the universe with a philosophical eye. I We may begin by considering a group of arguments based on largescale features of the history of life. In the course of this history there has occurred an immense multiplication of the different kinds of organisms which inhabit the earth. The classification of these kinds is the special province of taxonomy or systematics , and its scheme of categories is hierarchical in arrangement. At the base of the hierarchy is the category of species, of which approximately one million and a half are recognized today. The palaeontological evidence makes it fairly clear that nothing like this number of species existed when life was in its early stages. Near the top of the hierarchy is the category of phylum. Ouly about twenty phyla can be distinguished within the animal kingdom. Each of them represents a broad and fundamental form of animal organization, such as is exemplified in the protozoans , the annelids, the molluscs, the arthropods, and the vertebrates. Between the phyla and the species other systematic categories (e.g. classes, orders, families, genera, etc.) have their place, and each has been subject to numerical increase as evolution has proceeded. Now it is pointed out that the broadest forms of animal organization, the phyla, have all been in existence for a very long time. No new phylum has appeared for at least four hundred million years. Moreover, by no stretch of scientific imagination can we discern in any type of organism living at present the potentialities which might enable it to give rise to a new phylum in the future. Every type of organism is so specialized in structure and function that it is biologically impossible for changes of the magnitude reqnired to begin, let alone be completed. The conclusion here seems evident. Phyletic evolution not only is finished but was finished hundreds of millions of years ago. We cannot reasonably expect that the future course of life will require the addition of any fresh categories at the top of the taxonomic scale. What about...