158 Dr. W. Kiikentlial on the Adaptation of 



author the only new element necessary for the earliest origin 

 of the dolphin's caudal fin, as well as the attainment of its 

 highest perfection, consisted in rough embryonic variations, 

 which presented something approaching the form of a fin-like 

 appendage, so that therefore this new formation is to be 

 regarded as the product of individual selection in the Dar- 

 winian sense. The internal structure of the caudal fin of con- 

 nective tissue, which is so extraordinarily practical and 

 constructed according to mechanical laws, cannot, on the 

 other hand, be explained by means of selection alone ; it is a 

 functional adaptation, a specialization of the qualities of the 

 tissues. 



What we have thus far established is this : in consequence 

 of adaptation to the aquatic life the external form of the 

 mammals concerned assumes a special fish-like shape ; this 

 transformation is intensified in the case of those mammals 

 which remain permanently in the water, for in them a quicker 

 and more agile motion is obtained by the substitution of the 

 complicated lashing action of the tail for the oar-like action 

 of the extremities. Every function of the hind limbs is sus- 

 pended, and they therefore disappear, while the tail undergoes 

 that expansion by which its utility is increased, and so 

 becomes the caudal fin. In the case of the seals a transition 

 is observable, in so far as one division, the true seals, use the 

 posterior limbs in swimming in a similar manner to that in 

 which the whales use their tail. It may here suffice to state 

 that, from the standpoint of comparative anatomy, the attempt 

 has been made (Kyder)* to explain the caudal fin of the 

 whale and the hind feet of the seal as homologous structures. 



Thus we see how similarity ot outward form results from 

 similar adaptation, in the present instance to life in water ; and 

 in the various aquatic mammals, to which we have alluded, 

 we can trace this in all its varying stages. 



These phenomena appear more distinct and applicable as 

 direct proofs when we select separate systems of organs for the 

 purpose of exan)ination. It seems simplest to select the 

 external body-covering of the aquatic mammals for the pur- 

 pose of examination. We may state forthwith that great 

 difterences will be found between temporarily and permanently 

 aquatic mammals as regards the partial transformation of the 

 integument. Temporarily aquatic mammals, for instance, 

 cannot do without a covering of hair, which to the others 

 becomes a useless encumbrance. The changes which we 



* John Ryder, " On the Development of the Cetacea, together with a 

 Consideration of the probable Iloniohigies of the Flukes of Cetaceans and 

 Sirenians," United States Comm. of Fish and Fisheries, 1885, p. 427. 



