176 Dr. W. Kiikenthal on the Adaptation of 



value of the sections became much less, and we therefore see 

 •within the limits of each species a certain amount of variation 

 in their number. The tendency to the formation of small 

 skeletal divisions, to which the retarded ossification and the 

 formation of double epiphyses originally lent an impetus, was 

 at length carried so far that the fingers also began to split in 

 a longitudinal direction, as is seen in the much expanded 

 fifth digit of Odontocetes. In the whales the process of 

 modifying the skeleton of the flipper has not yet said its last 

 word ; we see how new double epiphyses are already arising 

 again in the secondary finger-joints and how in the case of a 

 toothed whale, Glolnocephalus^ the same process of increasing 

 the number of segments at the tip of the second finger has 

 already commenced anew, so that therefore it comes in this 

 case to the formation of tertiary phalanges. A portion of the 

 GJohiocepliali do not yet exhibit this process ; the majority, 

 on the other hand, have already acquired it. 



Precisely the same transformation of the skeleton was 

 experienced in earlier periods of the earth's history by the 

 hands of reptiles now extinct, the Plesiosauri and Ichthyo- 

 sauri, which likewise became adapted to the aquatic life. In 

 the case of the former the process ceased at a comparatively 

 early stage, much as it is seen in existing whalebone whales ; 

 the Ichthyosaurs, on the other hand, carried it much further. 

 "While the paddles of some Ichthyosauri (those of the older 

 forms) show the greatest similarity to those of the toothed 

 whales, in the more recent Ichthyosaurs the modification is 

 much greater. 



The transformation of the fore limb into the swimming- 

 paddle is therefore regulated by the same laws in widely 

 distant groups. Whalebone whales and toothed whales, 

 Plesiosauri and Ichthyosauri — four groups, not traceable to one 

 another, but originating from difterent terrestrial ancestors — 

 have acquired precisely similar anterior extremities as a result 

 of the operation of the same laws on the modification of the 

 fore limb. The phenomenon of convergence is here revealed 

 with the utmost distinctness. It appears as if the various 

 flippers were approaching a single ty])e, which has received 

 its fullest expression in the fin of the fish. 



Once again we arrive at the conviction, as we have already 

 done in considering the integument of the aquatic mammals, 

 that a large series of resemblances in the structure of the 

 flippers is but the result of convergence, and that it is a 

 mistake to bestow on them phylogenetic value. These must 

 be eliminated if we would compare the extremities of whale- 

 bone and toothed whales with one another. After we have 



