Evolution and Adaptation

by Thomas Hunt Morgan

“But perhaps the most discouraging circumstance of all is, that we can assert in scarcely a single actual instance in nature whether an observed variation is useful or not—a drawback that I distinctly emphasized some time ago. Nor is there much hope of betterment in this respect, for think how impossible it would be for us to observe all the individuals of a species in all their acts of life, be their habitat ever so limited—and to observe all this with a precision enabling us to say that this or that variation possessed selective value, that is, was a decisive factor in determining the existence of the species.”

“And thus it is everywhere. Even in the most indubitable cases of adaptation as, for instance, in that of the striking protective coloring of many butterflies, the sole ground of inference that the species on the whole is adequately adapted to its conditions of life, is the simple fact that the species is, to all appearances, preserved undiminished, but the inference is not at all permissible that just this protective coloring has selective value for the species, that is, if it were lacking, the species would necessarily have perished.”

Few opponents of Darwinism could give a more pessimistic account of the accomplishments of the theory of natural selection than this, by one of the leaders of the modern school: “Discouraging, therefore, as it may be that the control of nature in her minutest details is here gainsaid us, yet it were equivalent to sacrificing the gold to the dross, if simply from our inability to follow out the details of the individual case we should renounce altogether the principle of selection, or should proclaim it as only subsidiary, on the ground that we believe the protective coloring of the butterfly is not a protective coloring, but a combination of colors inevitably resulting from internal causes. The protective coloring remains a protective coloring whether at the time in question it is or is not necessary for the species; and it arose as protective coloring—arose not because it was a constitutional necessity of the animal’s organism that here a red and there a white, black, or yellow spot should be produced, but because it was advantageous, because it was necessary for the animal. There is only one explanation possible for such patent adaptations, and that is selection. What is more, no other natural way of their originating is conceivable, for we have no right to assume teleological forces in the domain of natural phenomena.”

Weismann states that he does not accept Eimers’s view that the markings of the wings of the butterflies of the genus Papilio are due to a process of evolution in a direct line, independent of external causes.

“On the contrary, I believe it can be clearly proved that the wing of the butterfly is a tablet on which Nature has inscribed everything she has deemed advantageous to the preservation and welfare of her creatures, and nothing else; or, to abandon the simile, that these color patterns have not proceeded from inward evolutional forces but are the result of selection. At least in all places where we do understand their biological significance these patterns are constituted and distributed over the wing exactly as utility would require.”

Again: “I should be far from maintaining that the markings arose unconformably to law. Here, as elsewhere, the dominance of law is certain. But I take it, that the laws involved, that is, the physiological conditions of the variation, here are without exception subservient to the ends of a higher power—utility; and that it is utility primarily that determines the kind of colors, spots, streaks, and bands that shall originate, as also their place and mode of disposition. The laws come into consideration only to the extent of conditioning the quality of the constructive materials—the variations, out of which selection fashions the designs in question. And this also is subject to important restrictions, as will appear in the sequel.” This conclusion contains all that the most ardent Darwinian could ask.

He rejects the idea that internal laws alone could have produced the result, because:—

“If internal laws controlled the markings on butterflies’ wings, we should expect that some general rule could be established, requiring that the upper and under surfaces of the wings should be alike or that they should be different, or that the fore wings should be colored the same as or differently from the hind wings, etc. But in reality all possible kinds of combinations occur simultaneously, and no rule holds throughout. Or, it might be supposed that bright colors should occur only on the upper surface or only on the under surface, or on the fore wings or only on the hind wings. But the fact is they occur indiscriminately, now here, now there, and no one method of appearance is uniform throughout all the species. But the fitness of the various distributions of colors is apparent, and the moment we apply the principle of utility we know why in the diurnal butterflies the upper surface alone is usually variegated and the under surface protectively colored, or why in the nocturnal butterflies the fore wings have the appearance of bark, of old wood, or of a leaf, whilst the hind wings, which are covered when resting, alone are brilliantly colored. On this theory we also understand the exceptions to these rules. We comprehend why Danaids, Heliconids, Euploids, and Acracids, in fact all diurnal butterflies offensive to the taste and smell, are mostly brightly marked and equally so on both surfaces, whilst all species not thus exempt from persecution have the protective coloring on the under surface and are frequently quite differently colored there from what they are on the upper.

“In any event, the supposed formative laws are not obligatory. Dispensations from them can be issued and are issued whenever utility requires it.”

Dispensations from the laws of growth! Does not a philosophy of this sort seem to carry us back into the dark ages? Is this the best that the Darwinian school can do to protect itself against the difficulties into which its chief disciple confesses it has fallen?