CHAPTER XX 



Fossil pedigrees Ancestry of birds, horses, elephants and whales. 



IN our last chapter we gave a brief outline of the general course 

 of evolution amongst vertebrate animals as indicated by the 

 geological record. We may now study in somewhat greater 

 detail certain branches of the great phylogenetic tree which are 

 especially well represented by fossil remains and therefore 

 particularly instructive from the point of view of the evolution 

 theory. 



One of the most highly specialized groups of vertebrates that 

 have ever existed is that of the birds. We have already pointed 

 out that, on anatomical grounds, birds are classed together with 

 reptiles as Sauropsida. They agree with reptiles in their method 

 of reproduction by means of large, heavily yolked eggs, and in 

 the presence, in the embryo, of the characteristic foetal membranes, 

 amnion and allantois, as well as in certain morphological charac- 

 ters of the adult. They differ from reptiles, however, in many 

 striking features. Thus they possess feathers, which almost (but 

 not quite) completely replace the reptilian scales as a protective 

 exo-skeleton. The anterior limbs are modified to form wings, 

 constructed, as we have already seen in Chapter XVII, on an 

 entirely different plan from those of flying reptiles. The digits of 

 the hand are very greatly reduced ; only one of them, and that in 

 a vestigial condition, projects freely from the anterior border of the 

 wing, forming the so-called " ala spuria " (Fig. 99). The true tail 

 is greatly abbreviated and the caudal vertebrae reduced in number 

 and to a large extent fused together to form the " ploughshare 

 bone " which supports the tail feathers. Lastly, all existing birds 

 have completely lost their teeth, which are functionally replaced 

 by the horny beak. 



The remains of the earliest known bird, Archaeopteryx (Fig. 151), 

 have been found in the celebrated lithographic stone of Solenhofen 

 in Bavaria, of Upper Jurassic age, which, owing to its extremely 

 fine grain, is peculiarly well suited for the preservation of even 



B. X 



