Morphology of the Fins 63 



tiveness of origin among structures. As to ontogeny, it is, of 

 course, true that through heredity " the Hfe-history of the indi- 

 vidual is an epitome of the Hfe-history of the race." " Onto- 

 geny repeats phylogeny," and phylogeny, or line of descent of 

 organisms and structures, is what we are seeking. But here 

 the repetition is never perfect, never nearly so perfect in fact as 

 Ha^ckel and his followers expected to find it. The demands 

 of natural selection may lead to the lengthening, shortening, 

 or distortion of phases of growth, just as they may modify 

 adult conditions. The interpolation of non-ancestral stages is 

 recognized in several groups. The conditions of the individual 

 development may, therefore, furnish evidence in favor of cer- 

 tain theories of origins, but they cannot alone furnish the abso- 

 lute proof. 



In the process of development the median or vertical fins 

 are doubtless older than the paired fins or limbs, whatever be 

 the origin of the latter. They arise in a dermal keel which is 

 developed in a web fitting and accentuating the undulatory 

 motion of the body. In the embryo of the fish the continuous 

 vertical fin from the head along the back and around the tail 

 precedes any trace of the paired fins. 



In this elementary fin-fold slender supports, the rudiments 

 of fin-rays, tend to appear at intervals. These are called by 

 Ryder ray-hairs or actinotrichia. They are the prototype of 

 fin-rays in the embryo fish, and doubtless similarly preceded 

 the latter in geological time. In the development of fishes 

 the caudal fin becomes more and more the seat of propulsion. 

 The fin-rays are strengthened, their basal supports are more 

 and more specialized, and the fin-fold ultimately divides into 

 distinct fins, the longest rays developed where most needed. 



That the vertical fins, dorsal, anal, and caudal, have their 

 origin in a median fold of the skin admits of no question. 

 In the lowest forms which bear fins these structures are dermal 

 folds, being supported by very feeble rays. Doubtless at first 

 the vertical fins formed a continuous fold, extending around 

 the tail, this fold ultimately broken, by atrophy of parts not 

 needed, into distinct dorsal, anal, and caudal fins. In the 

 lower fishes, as in the earlier sharks, there is an approach 

 to this condition of primitive continuity, and in the embryos 



