Evolution and Adaptation

by Thomas Hunt Morgan

Geoffroy’s views were largely influenced by his studies in systematic zoology and by his conception of a unity of plan running through the entire animal kingdom. His study of embryology and paleontology had led him to believe that present forms have descended from other organisms living in the past, and in this connection his discovery of teeth in the jaws of the embryo of the baleen whale and also his discovery of the embryonic dental ridges in the upper and in the lower jaws of birds, were used with effect in supporting the theory of change or evolution. Lastly, his remarkable work in the study of abnormal forms prepared the way for his conception of sudden and great changes, which he believed organisms capable of undergoing. He went so far in fact, in one instance, as to suppose that it was not impossible that a bird might have issued fully equipped from the egg of a crocodile. Such an extreme statement, which seems to us nowadays only laughable, need not prejudice us against the more moderate parts of his speculation.

His study of the fossil gavials found near Caen led him to believe that they are quite distinct from living crocodiles. He asked whether these old forms may not represent a link in the chain that connects, without interruption, the older inhabitants of the earth with animals living at the present time. Without positively affirming that this is the case, he did not hesitate to state that a transformation of this sort seemed possible to him. He said: “I think that the process of respiration constitutes an acquirement so important in the ‘disposition’ of the forms of animals, that it is not at all necessary to suppose that the surrounding respiratory gases become modified quickly and in large amount in order that the animal may become slowly modified. The prolonged action of time would ordinarily suffice, but if combined with a cataclysm, the result would be so much the better.”

He supposed that in the course of time respiration becomes difficult and finally impossible as far as certain systems of organs are concerned. The necessity then arises and creates another arrangement, perfecting or altering the existing structures. Modifications, fortunate or fatal, are created which through propagation are continued, and which, if fortunate, influence all the rest of the organization. But if the modifications are injurious to the animals in which they have appeared, the animals cease to exist, and are replaced by others having a different form, and one suited to the new circumstances.

The comparison between the stages of development of the individual and the evolution of the species was strongly impressed on the mind of Geoffroy. He says: “We see, each year, the spectacle of the transformation in organization from one class into another. A batrachian is at first a fish under the name of a tadpole, then a reptile (amphibian) under that of a frog.” “The development, or the result of the transformation, is brought about by the combined action of light and of oxygen; and the change in the body of the animal takes place by the production of new blood-vessels, whose development follows the law of the balancing of organs, in the sense, that if the circulating fluids precipitate themselves into new channels there remains less in the old vessels.” By preventing tadpoles from leaving the water, Geoffroy claims that it has been shown that they can be prevented from changing into frogs. The main point that Geoffroy attempts to establish is no doubt fairly clear, but the way in which he supposes the change to be effected is not so clear, and his ideas as to the way in which new change may be perpetuated in the next generation are, from our more modern point of view, extremely hazy. It is perhaps not altogether fair to judge his view from the standpoint of the origin of adaptive structures, but rather as an attempt to explain the causes that have brought about the evolution of the organic world.

During the remainder of the nineteenth century there accumulated a large number of facts in relation to the action of the external conditions in bringing about changes in animals and plants. Much of this evidence is of importance in dealing with the question of the origin of organic adaptation.

The first class of facts in this connection is that of geographical variation in animals and plants. It will be impossible here to do more than select some of the most important cases. De Varigny, in his book on “Experimental Evolution,” has brought together a large number of facts of this kind, and from his account the following illustrations have been selected. He says: “When the small brown honey-bee from High Burgundy is transported into Bresse—although not very distant—it soon becomes larger and assumes a yellow color; this happens even in the second generation.” It is also pointed out that the roots of the beet, carrot, and radish are colorless in their wild natural state, but when brought under cultivation they become red, yellow, etc. Vilmorin has noted that the red, yellow, and violet colors of carrots appear only some time after the wild forms have been brought under cultivation. Moquin-Tandon has seen “gentians which are blue in valleys become white on mountains.” Other cases also are on record in which the colors of a plant are dependent on external conditions.

The sizes of plants and animals are also often directly traceable to certain external conditions; the change is generally connected with the amount of food obtainable. “Generally speaking,” De Varigny says, “insular animals are smaller than their continental congeners. In the Canary Islands the oxen of one of the smallest islands are smaller than those on the others, although all belong to the same breed, and the horses are also smaller, and the indigenous inhabitants are in the same case, although belonging to a tall race. It would seem that in Malta elephants were very small,—fossil elephants, of course,—and that during the Roman period the island was noted for a dwarf breed of dogs, which was named after its birthplace, according to Strabo. In Corsica, also, horses and oxen are very small, and Cervus corsicanus, the indigenous deer, is quite reduced in dimensions; ... and lastly, the small dimensions of the Falkland horses—imported from Spain in 1764—are familiar to all. The dwarf rabbits of Porto Santo described by Darwin may also be cited as a case in point.”

These facts, interesting as they are, will, no doubt, have to be more carefully examined before the evidence can have great value, for it is not clear what factor or factors have produced the decrease in size of these animals.

The following cases show more clearly the immediate effect of the environment: “Many animals, when transferred to warm climates, lose their wool, or their hairy covering is much reduced. In some parts of the warmer regions of the earth, sheep have no wool, but merely hairs like those of dogs. Similarly, as Roulin notices, poultry have, in Columbia, lost their feathers, and while the young are at first covered with a black and delicate down, they lose it in great part as they grow, and the adult fowls nearly realize Plato’s realistic description of man—a biped without feathers. Conversely, many animals when transferred from warm to cold climates acquire a thicker covering; dogs and horses, for instance, becoming covered with wool.”

A number of kinds of snails that were supposed to belong to different species have been found, on further examination, to be only varieties due to the environment. “Locard has discovered through experiments that L. turgida and elophila are mere varieties—due to environment—of the common Lymnæa stagnalis.” He says, “These are not new species, but merely common aspects of a common type, which is capable of modification and of adaptation according to the nature of the media in which it has to live.” It has also been shown by Bateson that similar changes occur in Cardium edule, and other lamellibranchs are known to vary according to the nature of the water in which they live.