436 Causes and Course of Organic Evolution 



convergence above the notochord of the ventral lobes would 

 leave between them the fourth ventricle, while the lateral 

 nerves springing from these would be, as Hubrecht originally 

 contended, largely of a vagus and a glossopharyngeal char- 

 acter. 



So the fundamental conception in evolution of the main 

 invertebrate-vertebrate line of animal life as here sketched 

 is the manifold reception^ the concentration, and the correlation 

 of every form of energy that may environally affect the organism, 

 and the most satisfying response of the organism to this envir- 

 onment. 



Now, if such a dorsal uprising and backward extension of 

 the ventral ganglia in nemerteans gradually took place, we 

 might expect to find in cyclostomes and even higher verte- 

 brates all or some of the following structural relations: (1) 

 the dorsal mid area between the dorsal ganglia would be open 

 or covered only by a thin investing membrane; (2) the dorsal 

 mid area between the uprisen ventral ganglia would be even 

 more open; (3) the functions performed by the dorsal brain 

 masses as receptors and correlators of olfactory, optic, geo- 

 tactic, and auditory stimuli, also by the ventral masses as 

 receptors, correlators, and distributors of thermo-, thigmo-, 

 and chemotactic stimuli would be continued and even more 

 perfectly correlated; (4) as the uprisen ventral ganglia became 

 more closely applied to each other the lateral nerve cords 

 would become increasingly approximated, and on account of 

 the frequent transverse connecting fibers that joined each 

 would become in time symmetrical halves of a bilobed cord, 

 in which the connecting fibers remained as a bridge between 

 the more or less united halves; (5) receptor currents to, as 

 well as motor and inhibitory currents from, the cephalic gang- 

 lia would increasingly traverse this bilobed cord, which would 

 thus increasingly distribute the motor and inhibitory impulses 

 throughout the body; (6) some rudiment might be left of the 

 dorsal nemertean nerve, which starts from the dorsal com- 

 missure that joins the olfacto-optic areas of the dorsal ganglia, 

 and running backward distributes to the skin muscles accord- 

 ing to Burger (p. 108). 



All of the above conditions are fulfilled in Petromyzon, in 

 the Apoda, and upward through higher types in increasingly 

 perfected manner as one rises throughout the series. In this 

 process the areas between the ganglionic masses become the 

 brain ventricles, or the "iter" connections between these. 

 But a remarkable confirmation of the writer's position, as 

 well as of the earlier views of Hubrecht and of Harting, is 



