230 GRAND STRATEGY OF EVOLUTION 



the head, an essential condition to the growth and nu- 

 trition of the brain in man and in the higher verte- 

 brates. 



When we consider this sequence of events and con- 

 ditions in the evolution of the vertebrate heart, as re- 

 vealed to us by comparative anatomy and embryol- 

 ogy; observing in it the structural changes which fol- 

 low one another in due and logical order, as though in 

 response to physical forces; when we consider the 

 gradual rise of chambers and valves, inlets and out- 

 lets, mechanically correct in form and place for their 

 specific services; and the correlated rise and decline of 

 gills, the growth of lungs, and many other organs, who 

 shall say that we are not at the same time passing in 

 review the very cooperative agencies which bring them 

 about, if we only had the sense to discern them. 



How futile then it is, and essentially unscientific 

 in spirit, to ignore such factors because they are struc- 

 tural, or merely morphological, or phylogenetic ; and 

 to assume, as some apparently do, that it is simpler, or 

 easier, to explain them by the far-away god "vitality" 

 or "heredity," or by counting the beads on a rosary of 

 chromosomes ! 



3. The Blood. Blood is one of the most impor- 

 tant vital agencies of conveyance. Its constructive, or 

 creative, power is dependent not only on the arrange- 

 ment of its conducting channels, their nervous, mus- 

 cular, and physical attributes, but on what the blood 

 itself contains, its rate of movement, and its volume, 

 the direction in which it flows, and the way it ulti- 

 mately affects the life of the various kinds of cells it 

 serves. Here the improvements in service are not ex- 

 pressed in such obvious architectural features, but 



