Page:Popular Science Monthly Volume 66.djvu/237

Rh all the other tissues of the body are. Mutations are found arising according to partly known influences of kinship. They do not, so far as we observe, possess adaptive value when they first appear, but then frequently, if not always, develop into a stage of usefulness.

Fitness is, therefore, the central thought of modern paleontology in its most comprehensive sense, as embracing fitness in the very remote past, in its evolution toward the present and in its tendencies for the future. Just as the uniformitarian method of Lyell transformed geology, so the uniformitarian method is penetrating paleontology and making observations of animal and plant life as it is to-day the basis of the understanding of animal and plant life as it was from the beginning. Here again paleontology is not merely an auxiliary to zoology; it is chief of a division and enjoys certain unique advantages. We pass in review with the pedigrees and the prodigies of fitness, the entirely unreasonable, irrational, paradoxical extremes of structure, such, for example, as the pterosaurs, which far surpass in boldness and ingenuity of design any of the creations of the modern yacht builder which are mistakenly regarded by some as having reached an absurd extreme.

The paleontologist must also be a historian; he has to deal with lineage, with ancestors, he comes directly upon the problem of kinship or relationship, and he has to determine the various means of distinguishing the true from the apparent relationships. It happens that fitness, while fascinating in itself, has led even the most faithful and skilful into the most devious paths away from the truth. The explanation of this apparent contradiction is in this wise. The ingenuity of nature in adapting animals is astounding, but it is not infinite; the same devices are resorted to repeatedly to accomplish the same purposes. In the evolution of long-snouted rapacious swimming forms, for example, we have already discovered that nature has repeated herself twenty-four times in employing the same processes to accomplish the same ends in entirely different families of animals.

This introduces us to one of the two great ideas which we must employ in the interpretation of facts, namely, the idea of analogy. We see far more clearly than Huxley did the force of this idea. Owen, Cope, Scott, Fraas and many others', under the terms 'parallelism' 'convergence,' 'homoplasy' have developed the force of the old Aristotelian notion that analogy is a similarity of habit, and that in the course of evolution a similarity of habit finally results in a close or exact similarity of structure; this similarity of structure is mistaken as an evidence of kinship. Analogous evolution does not stop in its far reaching consequences with analogies in organs; it moulds animals as a whole into similar form, as, for example, the ichthyosaurs, sharks