PREFACE. xxm 



Marked deviations from the archetype characterising existing 

 species are directly approached in the progress of development. 



If, as, e.g., in a thoracic or jugular fish, the position of the 

 pelvic limbs departs from the typical one, these limbs bud out in 

 the embryo in that special and anomalous place. When a higher 

 species departs from type by a thoracic position of the scapular or 

 occipital limbs, they likewise bud out in such special position. In 

 both cases the haemal arch, sustaining such appendages, is libe- 

 rated from the rest of its segment for the special needs of the 

 species, and the embryo of such never shows it fixed. At most, 

 perhaps, the general character and typical connections may be 

 indicated by the closer contiguity of the detached scapular arch to 

 the rest of its proper occipital segment ; as, e.g., in the embryo of 

 birds and long-necked ruminants, to be removed to a distance 

 determined by the later growth of the series of vertebrae inter- 

 vening between head and chest. 



To infer from such developmental phenomena that the throat- 

 fins of the cod are not the displaced homologues of the hind legs 

 or pelvic limbs of air-breathers, and that the fore-legs of such are 

 not the homologues of the typically situated and connected scapu- 

 lar limbs of fishes, is an abuse or misuse of the empirical facts 

 ascertained by observation of embryonal phenomena. 



In like manner the developmental phenomena of the skull of 

 an avian and mammalian species, succeeding those that broadly 

 and intelligibly mark out the four pairs of neurapophyses and 

 corresponding haemal arches, plainly indicating the segmental or 

 vertebral type of the skull, depart therefrom to attain the par- 

 ticular character of the face and mouth of the species. After 

 the first budding indications of the halves of the maxillary (fore- 

 most cranial haemal) arch, the development of it, as upper jaw, 

 with that of the palate, pterygoid, and zygomatic appendages, 

 obeys the impress of impregnation, and proceeds directly to es- 

 tablish the specific characters of such jaw in the particular bird 

 or beast ; the points of ossification, their deposit in membrane or 

 gristle, and subsequent growth, having no other or deeper signifi- 



