The recapitulation theory.

Abstract

The so-called biogenetic law of Haeckel, that the developing organism, in its growth from the egg to the adult stage, recapitulates the past history of the race, was one of the most fascinating additions to the theory of evolution which was made during the nineteenth century. The publication of the first edition of the Origin of Species took place in 1859, and Haeckel's General Morphology, in which this law was enunciated, appeared in 1866. The belief in the validity of this law was the stimulating impulse to zoological research for over forty years. The enthusiasm evoked by it, particularly in the younger men of the time, can be understood when we reflect upon the clues actually available for following up the course of evolution. There are, in fact, three sources of evidence for the ancestry of existing forms, viz. (1) the comparative study of allied species, (2) the study of fossils, and (3) the study of development. The comparison with one another of allied species yields valuable information as to the most recent steps in evolution. When we find a uniform type of structure prevailing throughout almost all the members of a group of animals, and one or two exhibit divergences from this type, then we feel convinced that the type is older and that the divergences are recent developments of it. There is of course, and can be, no absolute proof that this is so; but the fact that all serious students of the animals in question feel unable to come to any other conclusion is presumptive proof enough for all reasonable men. Thus we feel sure that the air-breathing catfish of India (Clarias) with its curious pulmonary trees has been recently evolved from an ordinary type of cat-fish, that the flying-squirrel (Sciuroptems), with its parachute-like fold of skin extending between knee and elbow, is a development from the ordinary kind of squirrel. When, however, we attempt to follow the evolutionary process farther back, and connect together different orders or classes, our conclusions grow more and more uncertain. All such attempts implicitly assume that some group of animals has stood still, whilst an allied group has progressed in evolution. This may have happened, but it is utterly impossible to be sure that it has actually occurred. Attempts have been made to show that Mollusca, for instance, are descended from Platyhelminthes or from Sagitta. We can in imagination construct an intermediate series of types; but did such types actually exist? One zoologist thinks they may have existed, another that they did not, and the general feeling of those taking no part in the controversy is that no certainty at all can be attained on the subject, and that the further inquiry into such questions is waste of time. The study of fossils offers a much better promise of throwing light on the progress of evolution than the study of living forms. In certain favourable localities, especially in America, there are continuous series of beds thousands of feet thick and richly fossiliferous. We find the remains of a certain kind of animal in the lowest beds, and as we ascend in the series we find the remains of other animals closely allied to those in the basal beds, but diverging more and more as we approach the top of the series; and the conclusion is irresistibly forced on us that we are dealing with an evolutionary history. If, to take an instance, we examine the series of primitive horses of Oligocene date, we find that they all had three well-developed toes on both fore- and hind-feet. As we pass upwards in the series toward Miocene and Pliocene beds, the outer toes slowly diminish in size, until in modern horses they are represented by the vestigial splint bones hidden under the skin. Yet the process is so gradual, and immediately succeeding types resemble one another so closely, that as one eminent palaeontologist expressed it to me: I assure you that these lateral toes diminish in size only by millimetres at a time. …