230 Causes and Course of Organic Evolution 



kind or type of sensory stimulus. Tubes of varying energizing 

 stimuli would pass to these, and, on becoming linked within 

 them into a satisfied resultant response or responses, would 

 be radiated outward to appropriate centers of action. Such 

 is seen in diffuse state in several of the simpler metazoan groups. 



But, in the great line of evolutionary advance that in a later 

 chapter we trace from Infusoria through the rotifers, proto- 

 turbellarians, turbellarians, protonemertines, nemertines, pro- 

 tocyclostomes, and cyclostomes, we observe that from a small 

 multicellular brain in simpler rotifers we advance to such a 

 type as Callidina, in which an anterior olfactory and lateral 

 optic areas can be distinguished from the remainder of the 

 nervous or brain mass, that, from its nerve-thread connec- 

 tions, evidently combines a mechanotactic, a thermotactic, 

 and other sense functions. With increasing specialization and 

 aggregation in nerve substance the brain of a nemertean like 

 Eupolia is reached (p. 433). Here the superior or dorsal brain 

 lobes are primarily concerned ^ith correlation of the higher 

 cognitic stimuli, in that they are connected with strong ol- 

 factory or chemotactic, optic or helio tactic, and combined 

 audito-geotactic nerves. So the varied stimuli from without 

 that are passed by these nerves to the brain centers are there 

 combined into a resultant response. 



The above or dorsal again are connected with the ventral 

 brain lobes by transverse commissures. These ventral lobes 

 are primarily concerned with the reception of diffuse mechano- 

 tactic, thermotactic, and gustatory or chemotactic stimuli. 

 Such a nemertean brain therefore, from the standpoint of 

 proenvironmerit, is a complex sense-receptor, to which definite 

 sensory or energotactic nerves pass, wliich is itself composed 

 of definite sensory receptors and correlators, in which a definite 

 resultant response is built up, and from which a definite com- 

 pounded stimulus passes to a peripheral center or centers. 

 This stimulus then causes the animal to behave in an exact 

 and correlated manner to all of the incident stimuli. 



From such a brain to that of the elei)hant or ape the transi- 

 tion is gradual and progressive. Such transition involves 



