Mammals to Aquatic Life. 167 



Now it is apparently a deep and impassable gulf which 

 sepai'ates the fore limb of all other mammals from that of the 

 whalebone and toothed whales ; for while all other mammals 

 have three phalanges on each finger, with the exception of 

 the thumb, which has only two, both kinds of whales have 

 a larger number. This phenomenon is known as liyper- 

 phalangy, and no less than three hypotheses have been 

 formulated in recent times in order to explain it. According 

 to the first hypothesis the flippers of whales have no connexion 

 whatever with the fore limbs of land-mammals, but are ancient 

 organs which have been inherited from swamp-inhabiting 

 creatures [Leboucq). According to the second theory, the 

 supernumerary phalanges have arisen through the secondary 

 division of a strand of cartilage attached to the last phalanx, 

 such as has been stated to exist in the seals ( Weber ^ Ryder ^ 

 Baur). The tliird view is the one recently advanced by 

 Howes *, namely that the supernumerary phalanges arise 

 from intercalary syndesmoses, as in the Amphibia. 



It seems to me that none of these three hypotheses are 

 tenable, and in their stead I would suggest a fourth, namely 

 that the hyperphalangy is explicable by the process of double 

 epiphysis formation ; owing to the ever-increasing similarity 

 of the various parts and the retardation of ossification, the 

 epiphyses have attained a size equal to that of the diaphyses, 

 and have become equivalent to them. This change took 

 place a long time ago, and no longer admits of direct proof. 



I have previously t insisted on the fact that the entire 

 Cetacean finger corresponds to the typical Mammalian finger 

 and that the phalanges only are of different value. 



The question will now arise whether we have an instance 

 in nature of the way in which an increase of phalanges takes 

 place. If this process is actually going on anywhere, it must, 

 from what has gone before, be found among the aquatic 

 Mammalia; and it has actually been asserted by Baur | that 

 a fourth phalanx has been found in Sirenia. He writes : — 

 " Flower says, in the last edition of his ' Osteology of the 

 Mammalia,' that the number of phalanges in the tSirenia is 

 never increased beyond the limit usual in the Mammalia — 

 that is, three. But Dr. H. Gadow, in Cambridge, England, 



* G. B. Howes, " Observations upon the Morphology and Genesis of 

 Supernumerary Phalanj^es, -with especial reference to those of the Am- 

 phibia,'" Proc. Zoul. fSoc. Loudon, Dec. 4, 1888, p. 495. 



t Cf. Anat. Auzeif^^er, 1888, nos. 22 and 3U, and no. 2, 1800 ; also 

 Deukschiifton der medic.-uaturwiss. Gesellschaft, Jena, 1889, lid. iii. 



\ Baur, " On the Morphology and Origin of the lehthyopterygia," 

 American Naturalist, 1887, p. 840. 



