XXXVII -ON THE ORIGIN OF HETEROCERCY AND THE EVOLU- 

 TION OF THE FINS AND FIN-RAYS OF FISHES. 



By John A. Ryder. 



" AUes Gewordene, im Reiche der Natiir, ■wie in der Geschichte, iat niir durch sein "Werden zu be- 

 greifun wud dio Eiitwickelungsicceschiclito ist in diosem Sinn iiir den Niiturforscher vollkomnien das- 

 selbc, was dio Weltst'sfliicbte ftir die Menscbbeit."— Bruch : Wirbelthcorie des iichcidels. 



INTRODUCTORY. 



The following paper is the outgrowth of scattered observations made 

 (luring the last four years on the development of fishes, and in order to 

 make the results available, it has been thought best to bring them to- 

 gether in a permanent form. The portions which do not seem to the 

 reader to bear directly upon the theory of the tins, which it has been 

 sought to establish, may be regarded merely as collateral or supple- 

 mentary, and are introduced to further illustrate the kinetic or mechan- 

 ical hypotheses of- evolution of structural differentiation through the 

 voluntary or habitual movements of animals, Avhich the author has 

 l)reviously advocated in other publications in relation to other sets of 

 organs. Abstracts of some of the special results here published in full 

 have already appeared in another place.* 



The views here put forth in relation to the way iu which the morpho- 

 logical diiferentiatiou of the fins of fishes has occurred, rest partly 

 upon facts of ray own observation, but I must express my great indebt- 

 edness to the researches of L. and A. Agassiz, especially the latter, and 

 Vogt, Dohrn, Th. Lotz, Balfour, W. IST. Parker, Huxley, and Kolliker, 

 whose labors have prepared the way for me to co-ordinate many of the 

 known facts and establish doctrines founded upon the theory of onto- 

 geny, respecting the origin and differentiation of the fins, both ])aire<l 

 and unpaired. Acknowledgments are also due from me to Professor 

 Bairdforthe loan of valuable material from the collections of the National 

 Museum, and to Dr. Bean and Mr. G. Brown Goode, who have called 

 my attention to specimens which would otherwise have been overlooked. 

 Finally, I must express my thanks to Professor Gill for the interest he 

 has shown in calling my attention to extremely specialized types of fins 

 in rare or aberrant forms, and for aid in obtaining information upon the 

 literature of the subject. 



* 1. An outline of a theory of the development of the unpaired fins of fishes, Am. 

 Natnralist, 1885, pp. 90-97. 

 2. The development of the rays of osseous fishes, Am. Naturalist, 1885, pp. 200-204,, 

 [1] 981 



