314 



GENERAL PHYSIOLOGY 



ancestors which possessed the features in question. Of course the 

 record of comparative anatomy is also very incomplete, for existing 

 organisms are only the surviving tips of the various twigs of 

 the genealogical 'tree, between which the other twigs and 

 branches have perished. But here the palseontological record 

 supplements the facts of comparative anatomy up to a certain 



degree very satisfactorily, 

 by making the dead 

 branches accessible to 

 comparison with the still 

 living ones. An example 

 will illustrate this. Upon 

 comparative - anatomical 

 grounds the conviction 

 was formed that birds 

 stand in very close rela- 

 tionship to reptiles, but 

 forms that might be 

 considered as common 

 ancestors of the two or 

 were close to their ances- 

 tors were not known. 

 There was then dis- 

 covered in the quarries 

 of the lithographic slates 

 of Solenhofen a fossil 

 animal about the size of 

 a pigeon, the now well- 

 known ArcIicBopteryx muc- 

 rwrtts, which possessed the 

 characteristics of both 

 bird and reptile ; it had 

 the jaws of a lizard with 

 teeth, the spinal column 

 of a lizard, and a long 

 lizard-like tail ; but its 

 whole body was covered 

 with bird's feathers, which 

 were impressed upon the 

 rock most delicately (Fig. 134). By this and similar palasonto- 

 logical discoveries the kinship of the birds and the reptiles, which 

 was inferred from comparative anatomy, was very brilliantl}' con- 

 firmed. Similar examples may be cited in great number. 



Finally, embryology, or individual germinal development (onto- 

 geny), deals with the third important record of descent. As is 

 well known, the germs of plants and of animals from their simplest 

 condition, the egg-cell, pass through a long series of develop- 



'■J..L 



Fig. 134. — Archcto'pteryx macrums, s. lWiogra'phic%is. d, 

 Clavicle; co, coracoid ; h, humerus; r, radius; n, 

 ulna; c, carpus; sc, scapula; /. — IV. digits. (After 

 Zittel.) 



