Evolution and Adaptation

by Thomas Hunt Morgan

As an example of a simple action may be cited a well-known reflex after cutting the nerve-cord of the frog, or after destroying the brain. If the frog is held up, and its side tickled, the leg is drawn up to rub the place touched. To accomplish this requires a beautifully adjusted system of movements, yet the act seems to be a direct reflex, involving only the spinal cord.

An example of a somewhat more complex reflex is the biting off of the navel-string by the mother in rodents and other mammals; an act eminently useful to the young animal, although of no importance to the mother herself. The protection of the young by their parents from the attacks of other animals appears to be a somewhat complex instinct, and it is interesting to note that the protection is extended to the young only so long as they are in need of it, and as soon as they are able to shift for themselves the maternal protection is withdrawn.

The instinct of the young chick to seize in its beak any small moving object is a simple and useful reflex action, but if the object should happen to be a bee which stings the chick, another bee or similar insect will not be seized. Here we see that a reflex has been changed, and changed with amazing quickness. Moreover, the chick has learnt to associate this experience with a particular sort of moving object. It is this power to benefit by the result of a brief experience that is one of the most advantageous properties of the organism.

Young chicks first show a drinking reflex if by chance their beaks are wet by water. At once the head is lifted up, and the drop of water passes down the throat. In this way the chick first learns the meaning of water, and no doubt soon comes to associate it with its own condition of thirst. The sight of water produces no effect on the inexperienced chick, and it may even stand with its feet in the water without drinking; but as soon as it touches, by chance, the water with its beak, the reflex, or rather the set of reflexes is started.

A more complicated instinct is that shown by the spider in making its web. In some cases the young are born from eggs laid in the preceding summer, and can have had, therefore, no experience of what a web is like; and yet, when they come to build this wonderfully complex structure, they do so in a manner that is strictly characteristic of the species.

The formation of the comb by bees, in which process, with a minimum of wax, they secure a maximum number of small storehouses in which to keep their honey and rear their young, is often cited as a remarkable case of adaptation.

There has been some discussion as to whether birds build their nests in imitation of the nest in which they were reared, or whether they do so independently of any such experience. There can be no doubt, however, that in some birds neither memory nor imitation can play any important part in the result, and that they build their nests as instinctively as spiders make webs.

These instincts of spiders, bees, and birds appear to be more complex than the reflexes and tropisms that were first described. Whether they are really so, or only combinations of simple responses, we do not yet know. That they have come suddenly into existence as we now find them does not seem probable, but this does not mean that they must have been slowly acquired as the result of selection. The mutation theory also assumes that the steps of advance may have been small.

Our account may be concluded with the recital of some instincts, chosen almost at random, that serve to show some other adaptations which are the result of these inborn responses.

It is known that ants travel long distances from their nests, and yet return with unerring accuracy. It has been shown that they are able to do this through a marvellous sense of smell. The track left by the ant, as it leaves the nest, serves as a trail in returning to the starting-point. Moreover, it appears that the ant can pick out her own trail, even when it has been crossed by that of other ants. This means that she can distinguish the odor of her own trail from that of other members of the colony. The sense-organs by means of which the odor is detected lie in the antennæ. This fact accounts for certain actions of ants that have been described as showing that they have an affection for each other. Two ants, meeting, pat each other with their antennæ. In this way they are quickly able to distinguish members of their own nest from those of other nests. If they are of the same nest, they separate quietly; if of other nests, they may fight. If an ant from one nest is put into another nest, it is instantly attacked and killed—an act that appears to be injurious rather than useful, for the ant might become a valuable member of the new colony. If, however, an ant is first immersed in the blood of a member of the community into which she is to be introduced, she will not be attacked, and may soon become a part of the new community. By her baptism of blood she has no doubt acquired temporarily the odor of the new nest, and by the time that this has worn off she will have acquired this odor by association, and become thereby a member of another colony.