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For the present, therefore, we must conclude that feathers are not an adaptation, and not due to somatogenic modification, but must be result of a gametogenic mutation. Feathers, having been evolved, served in the wings and tail as important organs of flight.
Let us illustrate: In the giraffe of our former forest, one might appear whose neck was not longer because of slightly longer vertebrae, but who possessed an extra vertebrae. This would be a mutation. In other words, a mutation is a marked variation that may be inherited.
We find, in short, such evidence of the slow and scarcely sensible mutation of specific forms, as we have a just right to expect to find. There has been much discussion whether recent forms are more highly developed than ancient. I will not here enter on this subject, for naturalists have not as yet defined to each other's satisfaction what is meant by high and low forms.
No reversion accompanies mutation, and this fact is perhaps the completest contrast in which these two great types of variability are opposed to each other. The offspring of my mutants are, of course, subject to the general laws of fluctuating variability. They vary, however, around their own mean, and this mean is simply the type of the new elementary species.
Spencer begins by arguing that the constancy of human nature, so frequently alleged, is a fallacy. For change is the law of all things, of every single object as well as of the universe. "Nature in its infinite complexity is ever growing to a new development." It would be strange if, in this universal mutation, man alone were unchangeable, and it is not true.
"It is possible, without violating the laws of common sense, to establish a judgment whose terms will be modified by the mutation of causes. "But common sense demands that these different influences should be foreseen, and that these eventualities should be mentioned when pronouncing the judgment."
It seems to me that in the present state of our knowledge we cannot form a decisive opinion on the question whether the absence of limbs in such cases is the result of mutation or of disuse that is, absence of functional stimulation. The power of flight is an excellent example of adaptation. It has been evolved independently in Pterodactyls, Bats, and Birds.
On first considering the question of the origin of the rat-goat of Majorca, some naturalists will, no doubt, be tempted to suggest that it is a case of a sudden "sport," a "mutation" as they now call it, and not a result of gradual slowly developed reduction of the now lost teeth and correspondingly gradual enlargement of the two middle ones, taking many thousand generations to bring about.
The possibility of a Lamarckian explanation he does not even mention. He would doubtless assume that the antlers of stags arose as a mutation, without explaining how they came to be affected by the testicular hormone, and that when they arose the stags found them convenient as fighting weapons. But the complicated adaptive relations are not to be disposed of by the simple word mutation.
But he considers those mythological discourses about the gods as more persuasive and more adapted to truth, which assert that a divine nature is the cause of all good, but of no evil, and that it is void of all mutation, comprehending in itself the fountain of truth, but never becoming the cause of any deception to others. For such types of theology Socrates delivers in the Republic.
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