Evolution of Animals 489 



would affect slowly the main brain vesicles. But, as the impor- 

 tant researches of Johnston and Herrick have brought out, 

 it is- largely through increase in the olfactory region of the 

 first vesicle, and later the development of this as a center of 

 higher sensory or cognitic, and mental or cogitic, correlation, 

 that commencing growth and steady increase in the cerebral 

 lobes took place. Thus Herrick, speaking mainly of urodele 

 amphibians (162: 499) says: "Primitively the evaginated cere- 

 bral hemisphere was simply a primary and secondary olfactory 

 center. In very early phylogenetic stages ascending fibers 

 entered this secondary olfactory center from the pars dorsalis 

 thalami for olfac to-tactile correlation, etc., and from the hypo- 

 thalamus for olfacto- visceral correlations. 



"Since the dorso-medial part of the hemisphere is to a less 

 extent under the direct domination of any single one of the 

 functional systems which enter into the cerebral hemispheres, 

 in it the higher correlation tissue was first developed. The 

 preponderating element at first in this pallial correlating appa- 

 ratus was undoubtedly olfaction. Nevertheless cerebral cortex 

 is not developed under the influence of any single sensory 

 system, no matter how elaborately organized, and it is prob- 

 able that the primordium hippocampi, even in selachians and 

 amphibians, is concerned with the correlation of all the various 

 types of afferent impulses which reach the cerebral hemispheres 

 in these animals." 



Johnston again expresses (163: 457) a similar view. 



Further, as has been strikingly and perfectly pointed out 

 by Cope, Ryder, Osborn, and others, the whole osseous sys- 

 tem — teeth, skull, vertebrae, limbs — would show increasingly 

 perfect modification to suit perfecting response to varied envir- 

 onal stimuli that were passed into the brain centers. 



When we examine the very fragmentary record of premam- 

 malian fossil remains, and attempt to link these with living 

 types, certain broad results seem to the writer to be indicated. 

 Of existing mammals the marsupials, as already hinted, seem 

 to approach most nearly to an amphibio-mammalian origin. 

 For, from the triassic to the eocene period or even later, pre- 



16* 



