HIGHWAY OF ANIMAL EVOLUTION 227 



III. The Material and Functional Register of Vital 



Betterments 



A few examples will serve to illustrate some of the 

 different ways in which these organic readjustments are 

 actually made, the creative results which spring from 

 them, and the ways in which they are registered. 



I. The Old Mouth and the New. The closing 

 of the old invertebrate mouth and the formation in the 

 vertebrates of a new one to take its place is a case in 

 point, fig. 8. 



In the arachnids, the stomodaeum, or the oral end 

 of the gut, passes through the nervous system, between 

 the right and left roots of the forebrain. In the ver- 

 tebrates, it always lies outside the nervous system, and 

 wholly in front of the forebrain. What has doubt- 

 less happened is this. In the immediate ancestors of 

 the vertebrates, the old mouth became practically use- 

 less, owing to certain cumulative growth factors, and 

 a new one was formed out of an ancient, but transitory, 

 entrance to the alimentary canal, which originally 

 served a wholly different function. Many subsidiary 

 factors were involved in the event, and we can still see 

 some of them in actual operation in modern arachnids. 

 For example, the progressive constriction of the 

 oesophagus, by the growth of the surrounding brain, 

 ultimately compels all those with relatively larger 

 brains to suck their food in liquid form through the 

 narrowest possible opening, or give up eating alto- 

 gether. This clearly indicates that in this class of ani- 

 mals further enlargement of these two vital functions 

 has reached a mechanical impasse. On the other hand, 

 this now wholly useless oral opening still survives in 



