410 SCIENCE PROGRESS 



of the paired fins remain three in number, pre-, meso- and meta- 

 pterygium) but a rapid reduction must have been correlated 

 with the adaptation to progression on land (doubtless due to the 

 mechanical necessities of raising the body above the ground) so 

 as to reach the typical reduction characteristic of the Tetrapoda, 

 viz. to the single basal element, the double median element and 

 the pentadactylate distal portion, which is to be seen, already in 

 Carboniferous times, in such diagrammatic simplicity in the 

 Stegocephalan limb, e.g. Seeleya. At any rate pentadactylism 

 is a special instance of a very rapid reduction to a minimum 

 optimum. The horny fin-rays would conceivably coalesce at 

 the same time to form epidermal claws protecting the tips of the 

 distal radial elements of the limb. Thus there was a steady 

 system of progressive reduction until the Amphibian type was 

 reached. 



Just as a rapid reduction followed the adaptation of fins for 

 progression on land, so conversely when higher Vertebrates 

 have returned to an aquatic mode of life, the opposite tendency 

 is exhibited of extra or supernumerary digits being formed 

 adaptatively on the post-axial side of the limb. In the White 

 Whale or Beluga (Delphinapterus) this increase has been secured 

 by a longitudinal division of the fifth digit, and a splitting of 

 the digits took place in Ichthyosaurus. Hyperphalangism, or 

 the formation of additional phalanges, is clearly shown in aquatic 

 reptiles (Plesiosaurus, Ichthyosaurus) and also in mammals which 

 have become adapted to progression in water (Globicephalus and 

 other Cetaceans). In the Manatee {Manahis americaims) Baur^ 

 has shown that an extra (fourth) phalanx develops adaptively 

 on the third digit during the growth of the individual under 

 mechanical influence, whilst the embryos possess only the usual 

 three digits. The same tendency is exhibited by Chelonia, e.g. 

 in the Trionychoidca^ in which the fourth digit in each limb 

 never has less than four phalanges; in addition the members 

 of this group are losing their claws, which are unnecessary for 

 fin-like structures, only the three inner digits being provided 

 with them. In Carettochelys of New Guinea only two of 



Goodrich, Quart. Journ. Micr. Set. xlv. 1901) the pectoral and pelvic Hmbs clearly 

 show a reduction of the basal radials to a single element, supporting two elements 

 (one of which is regarded as a pre-axial radial), but at any rate this arrangement 

 gives us a strong hint of the Tetrapodan type. 

 ' Biolog. Caitralblatt, viii. 1887, p. 493. 



