260 STUDIES IN THE EVOLUTION OF ANIMALS 



In the Natural History Museum there is a fine collection of 

 Ichthyosaurs of the Mesozoic period. Nobody can doubt that their 

 four limbs, however rudimentary, as judged by our own hands and 

 feet, are the homologues of the four limbs of the higher vertebrates. 

 Well, in these Ichthyosaurs the digits of the hands, or what corre- 

 spond to digits, vary from four to eight. In several instances one 

 of the digits appears as if it were dwindling off. The carpal bones 

 are hardly distinguishable from the metacarpal and phalangeal 

 bones, which are very numerous, showing that in the higher verte- 

 brates the phalanges are reduced in number, either by suppression 

 or by coalescence. What is very notable is the fact that in some 

 the hind-legs are comparatively small, and with a reduced number 

 of digits. 



This may perhaps be the reason why some of the higher 

 vertebrates, such as the Tapir, Rhinoceros, and usually the Dog, 

 etc., have a reduced number of digits in their hind limbs. There 

 certainly does not seem to be any good reason for this reduction 

 which can be drawn from usefulness and natural selection. 



Here then in this Ichthyosaur plane of life we have a basis out 

 of which several archetypal hands and feet could have arisen. It 

 only needed that each monstrosity should have been repeated 

 through heredity, and made permanent by survival, as a race fit to 

 cope with its surrounding difficulties. 



In the Plesiosaurs, which apparently are closely allied to the 

 Ichthyosaurs, although they have a much longer neck, 1 we find 

 that the hand had already settled down from the many-digited to 

 the five-digited type, as shown in Fig. 93. And there would seem 

 little room for doubt that the Plesiosaur hand, or one like it, was 



1 Sprue of the Tortoises have a much longer neck than any of the Crocodiles ; and in 

 the snakes it is not easy to say where the neck commences and where it ends ; yet all 

 three sections are closely allied. 



