Evolution and Adaptation

by Thomas Hunt Morgan

It has been said that within each species the individuals differ more or less from each other, but our experience teaches that in each generation the same kinds of variations occur, and, moreover, that from any one individual there may arise in the next generation any one of the characteristic variations. Certain limitations will have to be made in regard to this statement, but for the present it will suffice. The Law of Biogenesis states that each living thing arises from another living thing; that there is no life without antecedent life, i.e. spontaneous generation does not occur. The law is not concerned with the likeness or unlikeness of the different individuals that descend from each other. The theory of evolution includes the same idea, but in addition it has come to mean nowadays, that there have been changes, as the succeeding generations have arisen. The transmutation theory, and even the descent theory, have come to mean nearly the same thing as the theory of evolution. It is unfortunate that one of these terms cannot be used to signify simply the repetition, generation after generation, of groups of similar individuals. The theory of descent might be used to convey only this idea, but unfortunately it too has come to include also the idea of change. I shall attempt nevertheless to discriminate between the descent and the transmutation theory, and use the term descent theory when I do not wish to convey the idea of change, and transmutation theory when I do wish to emphasize this idea.

On the transmutation theory it is assumed that a group (species) may give rise to one or more groups of forms differing from their ancestors; the original group being now replaced by its new kinds of offspring, or the old and the new may remain in existence at the same time. This process repeating itself, each or some of the new groups giving rise in turn to one or more new species, there will be produced a larger group of species having certain similar characters which are due to their common descent. Such a group of species is called a genus. The resemblances of these species is accounted for by their common descent; but their differences must be due to those factors that have caused them to depart from the original type. We may now proceed to consider the evidence on which this idea of transmutation rests.

Evidence in Favor of the Transmutation Theory

EVIDENCE FROM CLASSIFICATION AND FROM COMPARATIVE ANATOMY

It does not require any special study to see that there are certain groups of animals and of plants that are more like each other than they are like the members of any other group. It is obvious to every one that the group known as mammals has a combination of characters not found in any other group; such, for instance, as a covering of hair, mammary glands that furnish milk to the young, and a number of other less distinctive features. These and other common characteristics lead us to put the mammals into a single class. The birds, again, have certain common characters such as feathers, a beak without teeth, the development of a shell around the egg, etc., and on account of these resemblances we put them into another class. Everywhere in the animal and plant kingdoms we find large groups of similar forms, such as the butterflies, the beetles, the annelidan worms, the corals, the snails, the starfishes, etc.

Within each of these groups we find smaller groups, in each of which there are again forms more like each other than like those of other groups. We may call these smaller groups families. Within the families we find smaller groups, that are more like each other than like any other groups in the same family, and these we put into genera. Within the genus we find smaller groups following the same rule, and these are the species. Here we seem to have reached a limit in many cases, for we do not always find within the species groups of individuals more like each other than like other groups. Although we find certain differences between the individuals of a species, yet the differences are often inconstant in the sense that amongst the descendants of any individual there may appear any one of the other variations. If this were the whole truth, it would seem that we had here reached the limits of classification, the species being the unit. This, however, is far from being the case, for, in many species we find smaller groups, often confined to special localities. These groups are called varieties.

In some cases it appears, especially in plants, these smaller groups of varieties resemble in many ways the groups of species in other forms, since they breed true to their kind, even under changed conditions. They have been recognized as “smaller species” by a number of botanists.

In this connection a point must be brought up that has played an important rôle in all discussion as to what limits can be set to a species. As a rule it is found that two distinct species cannot be made to cross with each other, i.e. the eggs of an individual of one species cannot be fertilized by spermatozoa derived from individuals of another species; or, at least, if fertilization takes place the embryo does not develop. In some cases, however, it has been found possible to cross-fertilize two distinct species, although the offspring is itself more or less infertile. Even this distinction, however, does not hold absolutely, for, in a few cases, the offspring of the cross is fertile. It cannot be maintained, therefore, that this test of infertility between species invariably holds, although in a negative sense the test may apply, for if two different forms are infertile, inter se, the result shows that they are distinct species. If they cross they may or may not be good species, and some other test must be used to decide their relation.

We should always keep in mind the fact that the individual is the only reality with which we have to deal, and that the arrangement of these into species, genera, families, etc., is only a scheme invented by man for purposes of classification. Thus there is no such thing in nature as a species, except as a concept of a group of forms more or less alike. In nature there are no genera, families, orders, etc. These are inventions of man for purposes of classification.

Having discovered that it is possible to arrange animals and plants in groups within groups, the question arises as to the meaning of this relation. Have these facts any other significance than that of a classification of geometric figures, or of crystals according to the relations of their axes, or of bodies as to whether they are solids, liquids, or gases, or even whether they are red, white, or blue?