512 HISTORY OF THE HUMAN BODY 



anatomical speculation that necessary principle which had been 

 lacking in the philosophy of pre-Darwinian anatomists, and 

 from that time on actual ancestors took the place of theoretical 

 archetypes. It became then of fundamental importance to 

 establish the true interrelationships of the various animal 

 groups, that the structure of a given form might be explained 

 in terms of the ancestral structure of which it might be consid- 

 ered a modification. 



Naturally the intensest interest centered about the establish- 

 ment of the group from which the vertebrates were derived, 

 and here for a long time the most of the speculation followed 

 the lines laid down by St. Hilaire with his reversed insect. 

 Certainly one of the most characteristic features of a verte- 

 brate, and one of the earliest to appear in the embryo, is the 

 division of the body into somites, and the search for a bilateral 

 segmented ancestor must inevitably lead back to the articulates, 

 which alone of the invertebrates emphasize this characteristic to 

 an equal degree. It is true that in the two groups the arrange- 

 ment of the internal organs is in the main reversed, for in the 

 one the central nervous system is ventral and the main blood- 

 vessel dorsal, and in the other the former is dorsal and the lat- 

 ter ventral ; but the device employed by St. Hilaire to explain 

 this is by no means an absurd one, since what is called dorsal 

 or ventral in a given animal is merely its constant physiological 

 relation to the surface of the earth, and in several cases, like the 

 flounder and the squid, is known to be quite at variance with 

 the condition usual in related forms. 



Thus by postulating the occurrence of a change quite in ac- 

 cord with several known instances the differences in the rela- 

 tionships of the different systems in the simplest members of 

 both groups (annelids and selachians) may be brought into 

 almost complete harmony (Fig. 139, a and c). Even the noto- 

 chord, perhaps the greatest problem of vertebrate structure, 

 may be compared to the " Faserstrang," a bundle of fibers run- 

 ning along the nerve chain and serving as a support. This 

 and the notochord lie in a precisely similar position in relation 

 to the other organs, and in both cases they are enclosed with 



