228 GRAND STRATEGY OF EVOLUTION 



all modern vertebrates, including man, as an otherwise 

 meaningless passage-way through the floor of the brain 

 (infundibulum and saccus vasculosus), a permanent 

 register, in its proper place, of this great creative event. 

 p? and pt 2 . 



It is evident that immediate and far-reaching re- 

 sults are contingent on an open or shut factor of this 

 nature. It is not surprising, therefore, that the ulti- 

 mate shutting up of this passage-way should be coin- 

 cident with the onset of a great organic reformation, 

 and the abrupt appearance of a new class of animals, 

 such as the fish-like vertebrates. But it had still fur- 

 ther creative significance, for without this closing up 

 of what had come to be a very inconvenient gateway 

 to the gut, the growth of the brain, as we see it in the 

 higher vertebrates and in man, would have been a 

 physical impossibility. 1 



2. Gills, Lungs, and Heart. The rupture of gill 

 chambers into the alimentary canal was another sim- 

 ilar event, of immediate and far-reaching importance 

 in the evolution of fishes; and it still dominates their 

 internal economy, as well as that of the embryonic 

 stages in all their more modern descendants. 



But the gills of fishes, so long as they are retained 

 in their characteristic form and place, are an insuper- 

 able barrier to further evolution. Their position is 

 fundamentally wrong, from a mechanical standpoint, 

 both in relation to the heart and the alimentary canal, 

 and seriously restricts their further functional devel- 

 opment. For the delicate respiratory capillaries of the 

 gills are directly, and dangerously, interposed between 



1 For further details see "The Evolution of the Vertebrates aud Their 

 Kin." 



