HISTORICAL SKETCH 9 



invertebrate ancestors at a very remote period, before the special characters 

 which are typical of the higher classes of invertebrates and vertebrates 

 had been established. Many of the points of resemblance which have 

 been observed between the higher types of invertebrates and vertebrates 

 may be explained by assuming that certain fundamental characters, such 

 as a general bilateral symmetry involving the nervous system and sense- 

 organs, and certain common characters in the structure of the genito- 

 urinary system, have been retained in both. While the differences 

 between these classes may be accounted for on the assumption that a 

 gradual differentiation has arisen in the course of time in adaptation to 

 varying needs and possibilities of development, and that these changes, 

 taking place along divergent lines, have eventually led to the formation 

 of more fully evolved organs, which while retaining certain common 

 characters inherited from the ancestral stock, yet differ in important 

 respects, such as having eyes with an upright or an inverted type of retina. 

 Some idea of the antiquity of the vertebrate kingdom and, by inference, 

 the still greater antiquity of the common ancestor of the higher types of 

 invertebrates and vertebrates may be gained by a consideration of the 

 evidence which is afforded by the existence of the pineal organ in some of 

 the most ancient types of fossil fish, e.g. Cephalaspis, Pteraspis, and other 

 species. In these the small shallow pit which is situated between the 

 orbits and believed to mark the position of the pineal organ is on the inner 

 aspect of the cephalic shield. The existence of this depression on the 

 internal aspect of the vault of the skull indicates that even in these 

 archaic types of fish the pineal organ, although retaining its connection 

 with the skull, must have already been withdrawn from its primary super- 

 ficial position beneath the skin into the cavity of the skull, or that in the 

 course of ontogeny the " parietal organ " or " pineal eye " had been 

 severed from the stalk of the pineal outgrowth and that the distal end of 

 the latter had caused the impression on the inner aspect of the vault of 

 the skull, as is seen in many living types of fishes, Amphibia, and reptiles. 

 In either case it is evident that the organ in these extinct fishes must have 

 already reached a retrogressive or vestigial stage in its evolutionary history. 

 Since the " parietal organ " had already been cut off from its connection 

 with the brain as well as from the source of light, it is obvious that at this 

 very remote period in the history of the vertebrates it must have ceased 

 to function, at any rate as a light-perceiving organ. 



