[93] THE EVOLUTION OF THE FINS OF FISHES. 1073 



tails of fishes, natural selection is utterly incompetent to account for a 

 too numerous series of coincidences which afford the most imprej?uable 

 inductive basis for a theory of mechanical or dynamical evolution. The 

 rule that similar acts govern or are associated ^yith the production of 

 definite morphological modifications is too clearly made out to be broken 

 down by any a priori reasoning.* 



I have sought to show, in a paper on the laws of digital reduction, 

 how the expenditure of the energy dissipated in the act of locomotion, 

 if exerted by way of the middle series of digits, those finally usurped 

 the whole function of the original five, the lateral ones becoming rudi- 

 mientary. How in like manner the difference in the number of digits in 

 the fore and hind feet arose, assigning as a reason that whichever pair 

 was subjected to the severest strains was most apt to have the digits 

 reduced in number, as in the case of dogs, kangaroos, rabbits, &c., which 

 habitually make more or less rapid and successive bounds in running.! 



In now reverting to this subject, with a greater array of details in de- 

 fense of my position, I must admit that the views expressed by me five 

 years ago| were in accord with those expressed by Spencer in relation 

 to the genesis of bilateral symmetry, since shown to be not altogether 

 in harmony with the results of embryological research. The principle 

 then stated, that the alternate bending of the soft rays of fishes from 

 one side to the other led to their segmentation, I however still hold 

 to be true, for the reason that the primitive embryonic rays are never 

 segmented, but only after they have been blended and invested by 

 membrane substance, to form the matrix of permanent rays, and when 

 the muscles are already formed which move the latter, do the rays 

 show any evidence of transverse segmentation. This is the nearest 

 attainable approach to a demonstration that the alternate swinging 

 movement from side to side of the rays of fishes by the muscles has 

 to do with their transverse segmentation, since this segmentation is 

 always developed secondarily and after the membranous basis of the 



* Professor Cope, Science, IV, No. 87, 3:59, in an abstract of a paper by liiru on the 

 phylogeny of artiotlactyle Mammalia, " considered the derivation of the seleuodont 

 dentition, from the bunodont as established from a mechanical point of view." 



The evidence in favor of dental specialization of Mammals as modified by th© me- 

 chanical movements of the animals may be found in a series of papers by the writer, 

 published during the years 1877 to 1879, and based on observations made on the living 

 animals, as well as on the dentitions of living and extinct forms. See Am. Nat. 1877, 

 603. Nature, XVII, 1877, 128. On the mechanical genesis of tooth-forms, Proc. Acad. 

 Nat. Sci. Philada., 1878, 45-80. The significance of the diameter of the incisors in 

 rodents, Proc. A. N. S. Philada., 1877, 314-318. On the evolution and homologies of 

 the incisors of the horse, Proc. A. N. S. Philada., 1877, l.'')2-154. The mechanical 

 genesis of tooth-forms (Abstract by C.N. Pierce), Dental Cosmos, XX, 1878, 465-47-2. 

 Further notes on the mechanical genesis of tooth-forms, Proc. A. N. S. Philada., 1879, 

 47-51, and in a Review by E. D. Cope, Am. Nat., 1879, 446-449. 



t On the laws of digital reduction. Am. Naturalist, Oct., 1877, pp. 603-607. 



X On the origin of bilateral symmetry and the numerous segments of the soft rays 

 of fishes. Am. Naturalist, XIII, 1879, pp. 41-43. 

 H. Mis. 68 68 



