156 Dr. W. Kiikenthal on the Adaptation of 



Here lies, in my opinion, the chief weakness, from which all 

 these hypotheses suffer ; for we can quite easily conceive that 

 two animals which exhibit many points of structural agree- 

 ment may nevertheless not be allied to one another, but, as 

 branches of two perfectly distinct orders, have gradually 

 acquired similar characteristics through similar adaptation. 

 It shall be my task to prove this with special reference to the 

 hand of the aquatic Mammalia. 



It must be laid down from the outset as a fundamental 

 principle that all mammals living in water have sprung from 

 terrestrial forms. It is not merely considerations of a general 

 nature which lead to the advancement of this proposition ; 

 the proof has also been furnished in detail with the greatest 

 certainty, and there is scarcely anything new to add to it *. 



We will now, omitting a detailed account of the resem- 

 blances which have resulted from the adaptation to an aquatic 

 existence, devote our attention in the first place to a brief 

 general consideration of the external bodily form. It is 

 doubtless hardly necessary to draw especial attention to the 

 purely hypothetical nature of this method of examination. 



All water-mammals have acquired a more fish-like form 

 the longer and the more exclusively they have adapted them- 

 selves to the aquatic mode of life. There is no question 

 thereby of any retrogression ; the elongated, gradually dimin- 

 ishing form of body is the most practical for all vertebrates 

 which move by swimming. 



One section of the aquatic Mammalia is permanently con- 

 fined to the water ; another periodically spends a longer or 

 shorter portion of its life on land, and from this there arises a 

 highly important difference. In the case of those animals 

 which dwell for a time upon dry ground we shall find that 

 the extremities have not become so entirely adapted to the 

 functions of swimming as in the case of the others. Anterior 

 as well as posterior extremities are used upon the land as legs 

 and in the water as paddles, and the principal alteration which 

 we notice in animals of this class consists in an increase in 

 the size of the hind limb, which, for mechanical reasons, is 

 more utilized in swimming than the anterior extremity. 

 This difference in size may be often noticed in very pregnant 

 fashion in the case of the aquatic Mammals to which we have 

 alluded. 



In the case of those animals which remain permanently in 

 the water the function of the extremities as ambulatory organs 

 entirely disappears, and we are confronted with a modification 

 which is of great importance for locomotion in water, in that 



* For the Cetaceans see, for instance, Weber, " Ueber die cetoide Natur 

 der Promamuaalia," Anatoniischer Anzeiger, 1887, p. 42. 



