364 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



and a correspondingly diminished pressure would be 

 experienced by the now unused branchial portions of 

 the bows. The first would retain the importance of its 

 basal portion, as the source of the carotids, while the 

 middle arches would continue their existence as the 

 bases of the central dorsal aorta. The loss of the right 

 aorta-root in Mammalia was probably due to the fact 

 that the great arteries which supply the digestive sys- 

 tem are primitively branches of the left aorta-root, as 

 they are to-day in the crocodiles and in many of the 

 Batrachia. The right aorta-root disappeared through 

 disuse. Probably in the immediate ancestors of the 

 birds, as in the crocodiles, the right aorta-root gave 

 off the carotides and the subclaviae. As the birds de- 

 mand an excessive blood-supply for the fore limbs, we 

 have here probably the reason why the right root re- 

 mained in this subclass. 



The next index of successional development in Ver- 

 tebrata is the brain. Our belief that use under stimu- 

 lus has been the cause of its successive growth, can 

 only be based on the analogy of our own experiences 

 in the fnatter of education. No part of the human 

 organism is so susceptible to stimuli as the nervous 

 system, and the marvellous effects on faculty of con- 

 tinued exercise are well known to everybody. Since 

 the changes of mental states are necessarily due to 

 corresponding structural changes no one will find in 

 ignorance of the mechanics of brain-evolution a serious 

 obstacle to believing that it has taken place under the 

 influence of the innumerable stimuli always present to 

 animal life. 



It is in the skeleton that we have the actual record 

 and evidence of the effect of movement on structure. 

 It must be remembered in this connection that skeletal 



