PRESENT PROBLEMS OF PALEONTOLOGY. 233 



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. 



Problem of Historical Study. 



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 distin- 

 guishing 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 ex- 

 planation 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 re- 

 peated 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 Aris- 

 totelian 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 



VOL. LXVI. — 16. 



