vegetation sprang up on the lake bottom from seeds lying dormant there. One Winter a ditch was dug through a salt marsh, where the only higher plant was a species of marsh grassSpartina. The black peat cut from the ditch was piled in a ridge by its side so high that it was no longer covered by the tide. In the Spring various roadside weeds sprang up along the ridges, forming striking lines of vegetation running athwart the marsh. In these cases the germs were present, but failed to germinate until conditions suitable to their organization intervened. So, in general, there are abundant means of dissemination, and for almost every character there is a situation for which it is best suited. In that situation the new character will prove itself adapted to its environment.

THE MUTATION THEORY A KEY TO

DIFFICULTIES

The notion of mutation, when fully grasped, solves two difficulties which formerly confronted evolutionists. The first difficulty is the swamping effect of intercrossing. If the usual result of crossing a new character with its absence were a blend of the two conditions, then the difficulty would be a real one. But even in wild species any unit character typically fails to blend when crossed with its absence. The unit characters of violets, shepherd's purse, and spots of beetles are experimentally tested instances. The characters

of domesticated organisms behave in the same way, as illustrated by poultry. Unit characters, then, in so far as they refuse to blend, will not be swamped by intercrossing, but will reappear intact in a predictable proportion in successive generations.

The second difficulty which the mutation doctrine solves is discontinuity between species. Species differ in the presence or absence of certain unit characters. These unit characters are typically discontinuous in their origin. Hence it is futile to look for intergrades; as well might one look for intergrades between carbon monoxide and carbon dioxide. Species are discontinuous because specific characters are discontinuous; and specific characters, in so far as they are unit characters, are discontinuous because the molecular changes upon which they depend are discontinuous.

It is rash at the threshold of any new science to accept any one hypothesis to the exclusion of others. The president of our Association has taught us our duty toward multiple hypotheses. As in the newer chemistry transitions between molecules are becoming a recognized possibility, so it can not be denied that some unit characters may arise gradually; or, as a result of repeated crossing, show true blending and intergrading conditions. Many characters are indeed less or more because they have an ontogeny, and the adults stop at different points in the ontogeny,

as seems to be the case with human hair color. In many instances of geographic variation a gradation of climatic conditions causes a gradation in the development of a unit character all the way from invisibility to strong expression. Doubtless many important discoveries are about to be made in the field of graduated characters. But from henceforth we must, I think, start in our studies of unit characters from the standpoint of their normal discontinuity. While we remember the services of De Vries in insisting on the normal discontinuity of unit characters, we shall, in considering the idea of the unit character, recognize more clearly how great is the debt of biological science to the insight of Charles Darwin.

ADAPTATION

BY

CARL H. EIGENMANN

I. DEFINITIONS

THE chief object in the life of any animal is to leave another like it in its place when it dies. To this end we find numerous adjustments and compromises, adaptations in animals or plants, to place them in harmony with the elements of their physical or biological environment, or to coördinate the different parts of the same animal or plant.

We have major adaptations, such as those of birds, mammals, etc., for aërial respiration, and those of fishes for aquatic. We have also minor adaptations for a particular combination of temperature, light, heat, and the other elements of the physical environment. And, finally, we have adaptations fitting the animal to cope with other animals for a mate and a home, to secure food and to avoid being food.

Aside from adaptations an organism consists of vestiges, and frequently of other characters, that are not adaptations.

Vestiges, we know, are the remnants of past adaptations. Specific characters which are not

vestiges and are not now adaptations may also be past adaptations, or possibly they may become such in the future; it is only certain that they now do not particularly fit the species for survival. Some characters, while undoubtedly adaptive, give the impression that they are overdone. The antlers of the deer, the fang of the saber-tooth, the power of continuous growth of the incisors of rodents, are all adaptations that have in some instances proved to be too much of a good thing.

II. QUESTIONS

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In the words of Weismann, the most ardent of the Darwinians, Adaptations arise whenever needed if they are at all possible."

Adaptations have usually been looked upon as adjustments in the organism to its environment. The suggestion has more recently been made that adapted environments and habits are selected by animals adjusted to them.

Is a man healthy and strong because he practises athletics, or is he practising athletics because his strength inclines him to athletic sports? We have all been modified by our environment and by our activities. It is at least suggestive that some of us have never taken to pole-vaulting and should not have made a record if we had. Evidently there is a difference between the questions of the origin of adaptations in the individual and the origin of an adapted fauna.