Evolution and Adaptation

by Thomas Hunt Morgan

One interpretation of these facts is that the ancestral adult condition is repeated by the embryo, but as I have pointed out above in the cases of the teeth in whales, since the teeth do not reach the adult form, and do not even break through the gums in some forms, it is obviously stretching a point to claim that an adult condition is repeated. Moreover, in the case of the birds only the dental ridges appear, and it is manifestly absurd to claim in this case that the ancestral adult condition of the reptiles is repeated.

That a supposed ancestral stage may be entirely lost in the embryo of higher forms is beautifully shown in the development of some of the snakes. The snakes are probably derived from lizardlike ancestors, which had four legs, yet in the development the rudiments of legs do not appear, and this is the more surprising since a few snakes have small rudimentary legs. In these, of course, the rudiments of legs must appear in the embryo, but in the legless forms even the beginnings of the legs have been lost, or at any rate very nearly so.

Outside the group of vertebrates there are also many cases that have been interpreted as embryonic repetitions of ancestral stages, but a brief examination will suffice to show that many of these cases are doubtful, and others little less than fanciful. A few illustrations will serve our purpose. The most interesting case is that given by the history of the nauplius theory.

The free-living larva of the lower crustaceans—water-fleas, barnacles, copepods, ostracods—emerges from the egg as a small, flattened oval form with three pairs of appendages. This larva, known as the nauplius, occurs also in some of the higher crustaceans, not often, it is true, as a free form, as in penæus, but as an embryonic stage. The occurrence of this six-legged form throughout the group was interpreted by the propounders of the nauplius theory as evidence sufficient to establish the view that it represented the ancestor of the whole group of Crustacea, which ancestor is, therefore, repeated as an embryonic form. This hypothesis was accepted by a large number of eminent embryologists. The history of the collapse of the theory is instructive.

It had also been found in one of the groups of higher crustaceans, the decapods, containing the crayfish, lobster, and crabs, that another characteristic larval form was repeated in many cases. This larva is known as the zoëa. It has a body made up of a fused head and thorax carrying seven pairs of appendages and of a segmented abdomen of six segments. The same kind of evidence that justified the formulation of the nauplius theory would lead us to infer that the zoëa is the ancestor of the decapods. The later development of the zoëa shows, however, that it cannot be such an ancestral form, for, in order to reach the full number of segments characteristic of the decapods, new segments are intercalated between the cephalothorax and abdomen. In fact, in many zoëas this intercalated region is already in existence in a rudimentary condition, and small appendages may even be present. A study of the comparative anatomy of the crustaceans leaves no grounds for supposing that the decapods with their twenty-one segments have been evolved from a thirteen-segmented form like the zoëa by the intercalation of eight segments in the middle of the body. It follows, if this be admitted, and it is generally admitted now, that the zoëa does not represent an original ancestral form at all, but a highly modified new form, as new, perhaps, as the group of decapods itself. We are forced to conclude, then, that the presence of a larval form throughout an entire group cannot be accepted as evidence that it represents an ancestral stage. We can account for the presence of the zoëa, however, by making a single supposition, namely, that the ancestor from which the group of decapod has evolved had a larva like the zoëa, and that this larval form has been handed down to all of the descendants.

The fate of the zoëa theory cast a shadow over the nauplius theory, since the two rested on the same sort of evidence. The outcome was, in fact, that the nauplius theory was also abandoned, and this was seen to be the more necessary, since a study of the internal anatomy of the lowest group of crustaceans, the phyllopods, showed that they have probably come directly from many segmented, annelidian ancestors. The presence of the nauplius is now generally accounted for by supposing that it was a larval form of the ancestor from which the group of crustaceans arose.

The most extreme, and in many ways the most uncritical, application of the recapitulation theory was that made by Haeckel, more especially his attempt to reduce all the higher animals to an ancestral double-walled sac with an opening at one end,—the gastræa. He dignified the recapitulation theory with an appellation of his own, “The Biogenetic Law.” Haeckel’s fanciful and extreme application of the older recapitulation theory has probably done more to bring the theory into disrepute amongst embryologists than the criticisms of the opponents of the theory.

In one of the recognized masterpieces of embryological literature, His’s “Unsere Körperform,” we find the strongest protest that has yet been made against the Haeckelian pretension that the phylogenetic history is the “cause” of the ontogenetic series. His writes: “In the entire series of forms which a developing organism runs through, each form is the necessary antecedent step of the following. If the embryo is to reach the complicated end-forms, it must pass, step by step, through the simpler ones. Each step of the series is the physiological consequence of the preceding stage and the necessary condition for the following. Jumps, or short cuts, of the developmental process, are unknown in the physiological process of development. If embryonic forms are the inevitable precedents of the mature forms, because the more complicated forms must pass through the simpler ones, we can understand the fact that paleontological forms are so often like the embryonic forms of to-day. The paleontological forms are embryonal, because they have remained at the lower stage of development, and the present embryos must pass also through lower stages in order to reach the higher. But it is by no means necessary for the later, higher forms to pass through embryonal forms because their ancestors have once existed in this condition. To take a special case, suppose in the course of generations a species has increased its length of life gradually from one, two, three years to eighty years. The last animal would have had ancestors that lived for one year, two years, three years, etc., up to eighty years. But who would claim that because the final eighty-year species must pass necessarily through one, two, three years, etc., that it does so because its ancestors lived one year, two years, three years, etc.? The descent theory is correct so far as it maintains that older, simpler forms have been the forefathers of later complicated forms. In this case the resemblance of the older, simpler forms to the embryos of later forms is explained without assuming any law of inheritance whatsoever. The same resemblance between the older and simpler adult forms, and the present embryonic forms would even remain intelligible were there no relation at all between them.”

Interesting and important as is this idea of His, it will not, I think, be considered by most embryologists as giving an adequate explanation of many facts that we now possess. It expresses, no doubt, a part of the truth but not the whole truth.

We come now to a consideration of certain recently ascertained facts that put, as I shall try to show, the whole question of embryonic repetition in a new light.