28 THE RIDDLE OF THE UNIVERSE. 



It is quite true that all the vertebrates, from the fish 

 up to man, agree in every essential feature ; they all 

 have a firm internal skeleton, a framework of cartilage 

 and bone, consisting principally of a vertebral column 

 and a skull ; the advanced construction of the latter 

 presents many variations, but, on the whole, all may 

 be reduced to the same fundamental type. Further, 

 in all vertebrates the " organ of the mind," the central 

 nervous system, in the shape of a spinal cord and a 

 brain, lies at the back of this axial skeleton. More- 

 over, what we said of its bony environment, the skull, 

 is also true of the brain — the instrument of con- 

 sciousness and all the higher functions of the mind ; 

 its construction and size present very many variations 

 in detail, but its general characteristic structure 

 remains always the same. 



We meet the same phenomenon when we compare 

 the rest of our organs with those of the other verte- 

 brates ; everywhere, in virtue of heredity, the original 

 plan and the relative distribution of the organs remain 

 the same, although, through adaptation to different 

 environments, the size and the structure of particular 

 sections offer considerable variation. Thus we find 

 that in all cases the blood circulates in two main 

 blood-vessels, of which one — the aorta — passes over 

 the intestine, and the other — the principal vein — 

 passes underneath, and that by the broadening out of 

 the latter in a very definite spot a heart has arisen 

 this "ventral heart" is just as characteristic of all 

 vertebrates as the " dorsal heart " is of the articulata 

 and mollusca. Equally characteristic of all verte- 

 brates is the early division of the intestinal tube into 

 a " head-gut " (or gill-gut), which serves in respiration, 

 and a "body-gut" (or liver-gut), which co-operates 



