[91] THE EVOLUTION OF THE FINS OF FISHES. 1071 



diverged are they thrown out of the direct line of descent, as in the 

 cases of the Anurous Batrachiaus and Teleostean fishes, for example, 

 each of which is an example of a high state of evolution, and mark 

 extreme adaptations, which cannot therefore have any very close affilia- 

 tions with other series. Besides all this, the record of the process is so 

 blurred by adventitious adaptations, in consequence of the interaction 

 of organs upon each other through substitution of function, inducing 

 degeneration, or by special adaptive modifications, that real affinities 

 are often much obscured and rendered difficult of determination. 



Degeneration or abortion of certain structures, as well as the degener- 

 ation or degradation of a whole organism, not unfrequently results from 

 such causes as involve the disuse of a certain set of structures and their 

 atrophy, or extensive modification and changes in adjacent structures. 

 This may be illustrated by the fact that vertebral segments are clearly 

 indicated in the fore part of the chorda of the embryos of Amiurus^ 

 where five of them subsequently coalesce and a single compound seg- 

 ment is formed, due to a loss of mobility enforced by processes of the 

 post- temporal bones coming into contact with the lateral processes of 

 the anterior vertebrae. This also results in causing the abortion of 

 about the same number of myomeres in this region and their confluence 

 into a great muscular mass without any intermyomeral septa developed 

 between them. This also reacts upon some of the ribs, which are alsa 

 confluent or developed as a continuous osseous plate in the rib-forming 

 layer of the embryo. The anteriorly bifurcate air-bladder encroaches, 

 as it becomes larger upon the lower end of the anterior myotomes, the 

 pressure thus caused seeming to induce the abortion of the lower end 

 of the latter at this point. The air-bladder, as it grows out from the 

 fore-gut, finally has its parietes brought almost in contact with the 

 integument just behind the pectoral arch, where it may act as a tym- 

 panic organ in connection with the internal ear, as already suggested 

 by T. J. Parker in regard to a similar structure found in front of tbe 

 pectoral arch of the Red Cod. In very young embryos of Amiurus, or 

 those of the second day, the muscular segments are developed imme- 

 diately behind the pectoral plate and fold in just the same way and as 

 strongly as in the young Salmon of the same relative age, in the adult 

 of which there is no such abortion of the muscular side walls of the 

 anterior part of the visceral cavity. We have thus seen what a remark- 

 able series of modifications result correlatively from a change in the 

 mode of development of one or two adjacent parts. 



As the results of direct pressure exerted rhythmically we have the 

 superficial layers of cartilage cells overlying the articular ends of the- 

 bones of the limbs flattened, as Professor Leidy long ago suggested in 

 explanation of this condition. 



In the case of the dental armature of the Mammalia, I have sought to 

 show that the crowns of the teeth, especially in the Herhivora, have 

 been altered by use on the supposition that the enamel and dentine 



