Image from page 129 of "Water reptiles of the past and present" (1914) (14772688782)

Identifier: waterreptilesofp1914will

Title: Water reptiles of the past and present

Year: 1914 (1910s)

Authors: Williston, Samuel Wendell, 1851-1918

Subjects: Aquatic reptiles

Publisher: Chicago, Ill., The University of Chicago Press

Contributing Library: Boston Public Library

Digitizing Sponsor: Boston Public Library

Text Appearing Before Image:

are seen. So great are these differences that it hasbeen a puzzle to naturalists to understand how they could havearisen. In no other animals above the fishes, that is, in no otherreptiles, in no amphibians, birds, or mammals, are there ever morethan five fingers or five toes, the number with which air-breathinganimals began. Fingers and toes may be lost and often are lostin all groups of life, until a single one in each limb may remain, asin the domestic horse. An increase of fingers and toes, however,seems to be an impossibility in evolution, and doubtless of real n8 WATER REPTILES OF THE PAST AND PRESENT fingers and toes it is an equal impossibility. All naturalists are nowagreed that a specialized character can never revert to a generalizedcondition, or rather to a generalized structure, that an organ oncefunctionally lost can never be regained by descendants. A char-acter once lost is lost foreover; horses of the future can never havemore than one finger or one toe in each limb.

Text Appearing After Image:

Fig. 56.—Front paddle of Oph-thalmosaurus (after Andrews): h,humerus; r, radius; u, ulna; p,pisiform; re, radiale; int, inter-medium; tie, ulnare. r Fig. 57.—Front paddle of Mer-riamia, a Triassic ichthyosaur.(After Merriam.) Explanationsas in Fig. 56. And there was an increase in the ichthyosaurs, in some notonly of the number of digits in each limb, but in all of the numberof bones in each digit, a character found also in the unrelatedmosasaurs and plesiosaurs. This increase in finger and toe bones,or hyperphalangy as it is called, is one of the most peculiar of allthe adaptations to water life, changing the feet and hands fromthe ordinary walking type to the fish-like swimming type. Thebones beyond the humerus and femur in the ichthyosaurs were so ICHTHYOSAURIA no increased in number and so changed in form and relations thatthey bear little resemblance to the corresponding bones of otherreptiles. They are merely polygonal platelets of bone, articulatingon all sides and fitting

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