Evolution and Adaptation

by Thomas Hunt Morgan

There seems to be in all these cases an apparent connection between the absence of the digestive tract and the presence of an abundant supply of food, that has already been partly digested by the host. Put in a different way, we may say that the presence of this food has furnished the environment in which an animal may live that has a rudimentary digestive tract.

An interesting case of degeneration is found in the rudimentary mouth parts of the insects known as May-flies, or ephemerids. Some of these species live in the adult condition for only a few hours, only long enough to unite and deposit their eggs. In the adult stage the insects do not take any food. In this case the degeneration is obviously not connected with the presence of food, but apparently with the shortness of the adult life.

One of the most familiar cases of degeneration is blindness, associated with life in the dark. The most striking cases are those of cave animals, but this is only an extreme example of what is found everywhere amongst animals that live concealed during the day under stones, etc. The blind fish and the blind crayfish of the Mammoth Cave, the blind proteus of the caves of Carniola, the blind mole that burrows underground, the blind larvæ of many insects that live in the dark, are examples most often cited. Some nocturnal animals, like the earthworm, have no eyes, although they are still able to distinguish light; and some of the deep-sea animals, that live below the depth to which light penetrates, have degenerate eyes. The workers of some ants, that remain in the nests, are blind, but the males and the queens of these forms have well-developed eyes, although the eyes may be of use to them at only one short period of their life, namely, at the time of the marriage flight. This fact is significant and is underestimated by those who believe that disuse accounts for the degeneration of organs.

The wings of the ostrich and of the kiwi are rudimentary structures no longer used for flight, and many insects, belonging to several different orders, have lost their wings, as seen in fleas, some kinds of bugs, and moths, and even in some grasshoppers.

A curious case of degeneration is found in the abdomen of the hermit crab, which is protected by the appropriated shell of a snail. The appendages of one side of the abdomen have nearly disappeared in the male, although in the female the abdominal appendages are used to carry the eggs as in other decapod crustaceans. The abdomen, instead of being covered by a hard cuticle, as in other members of this group, is soft and unprotected except by the shell of the snail.

Cases of these kinds could be added to almost indefinitely, and the explanation of these degenerate structures has been a source of contention amongst zoologists for a long time. The most obvious interpretation is that the degeneration has been the result of disuse. But as I have already discussed this question, and given my reasons for regarding it as improbable that degeneration has arisen in this way, we need not further consider this point here.

The selectionists have offered several suggestions to account for degeneration. In fact, this has been one of the difficulties that has given them most concern. They have suggested, for example, that when an organ is no longer of use to its possessor it would become a source of danger, and hence would be removed through natural selection. They have also suggested that since such organs draw on the general food supply they would place their possessor at a disadvantage, and hence would be removed. Weismann has attempted to meet the difficulty by his theory of “Panmixia,” or universal crossing, by which means the useless structures are imagined to be eliminated.

These attempts will suffice to point out the straits to which the Darwinians have found themselves reduced, and we have by no means exhausted the list of suggestions that have been made. Let us see, if, on any other view, we can avoid some of the difficulties that the selection theory has encountered.

In the first place we shall be justified, I think, in eliminating competition as a factor in the process, since the admission that an organ has become useless carries with it the idea that it has no longer a selective value. If, in its useless condition, it is no longer greatly injurious, as is probably, though not necessarily always, the case, then selection cannot enter into the problem. If in parasitism we assume that an animal finds a lodgement in another animal, where it is able to exist, we may have the first stage of the process introduced at once. If under these conditions a mutation appeared, involving some of the organs that are no longer essential to the life of the individual in its new environment, the new mutation may persist. We need not suppose that the original form becomes crowded out, but only that a more degenerate form has come into existence. As a matter of fact we find in most groups, in which degenerate forms exist, a number of different stages in the degeneration in different species. Mutation after mutation might follow until many of the original organs have disappeared. The connection that appears to exist between the degeneration of a special part and the environment in which the animal lives finds its explanation simply in the fact that the environment makes possible the existence of that sort of mutation in it. We do not know, as yet, whether through mutative changes an organ can completely disappear, although this seems probable from the fact that in a few cases mutations are known to have arisen in which a given part is entirely functionless. If we could assume that, a mutation in the direction of degeneration being once established, further mutations in the same direction would probably occur, the problem would be much simplified; but we lack data, at present, to establish this view.

In the case of blind animals it seems probable that the transition has taken place in such forms as had already established themselves in places more or less removed from the light. Such forms as had the habit of hiding away under stones, or in the ground, living partly in and partly out of the light, might, if a mutation appeared of such a sort that amongst other changes the eyes were less developed, still be capable of leading an existence in the dark, while it might be impossible for them to exist any longer with weakened vision in the light. If such a process took place, the habitat of the new form would be limited, or in other words it would be confined to the locality to which it finds itself adapted; not that it has become adapted to the environment through competition with the original species, or, in fact, with any other.