January 7, 1910] 



SCIENCE 



hand, have a ladder-like nervous system 

 ventrally of the gut and of totally different 

 origin. 



These are illustrations of the nature of 

 the data on the basis of which zoologists 

 are in quite general agreement in recog- 

 nizing the wide divergence of these two 

 great phyla of metazoa. 



Now, students of animal behavior have 

 recognized also two fundamental behavior 

 types among the higher animals. This is 

 clearly stated by Yerkes when he points 

 out' that the animal kingdom presents di- 

 vergent lines in the development of action 

 types. 



Certain animals are marlcedly plastic or volun- 

 tary in their behavior, others are as markedly 

 fixed or instinctive. In the primates plasticity 

 has reached its highest known stage of develop- 

 ment; in the insects fixity has triumphed, in- 

 stinctive action is predominant. The ant has 

 apparently sacrificed adaptability to the develop- 

 ment of ability to react quickly, accurately and 

 uniformly in a certain way. Roughly, animals 

 might be separated into two classes: those which 

 are in high degree capable of immediate adapta- 

 tion to their conditions, and those which are 

 apparently automatic since they depend upon 

 instinctive tendencies to action instead of upon 

 rapid adaptation. 



If time permitted us to develop this con- 

 ception, many striking illustrations might 

 be cited of the predominance of now one, 

 now the other, of these action types in 

 different animals. The most striking fea- 

 ture of such an examination is the dis- 

 covery that, while the generalized members 

 of both of the zoologists' phyla exhibit an 

 extreme development of neither action 

 type, those forms which are structurally 

 highly specialized generally have one or 

 the other action type also highly devel- 

 oped; and in these cases an arrangement 

 of animals according to their type of be- 

 havior follows closely the diphyletic ar- 



^ Journal of Comparative Neurology and Psy- 

 chology, Vol. 15, 1905, p. 137. 



rangement previously elaborated on purely 

 structural grounds. The anatomical basis 

 of this harmony is readily apparent when 

 the nervous systems of the two phyla are 

 compared. 



The central nervous system of the articu- 

 lates is fundamentally a segmented ladder- 

 like chain of nerve tissue with special gan- 

 glionic enlargements in the head related to 

 the leading sense organs. It is dominated 

 by the general body metamerism and the 

 segmental reflex arcs are kept relatively 

 distinct by the anatomical configuration. 

 Some of the compound and chain reflexes 

 are very complex; yet they tend to follow 

 the appropriate stimuli with a mechanical 

 precision which is simply an expression of 

 the accurate working of a pre-formed 

 mechanism. Since this is an inherited 

 mechanism, all members of the species 

 exhibit similar reactions and these do not 

 require experience for their performance. 

 This is instinct. 



On the other hand, the vei'tebrate nerv- 

 ous system is fundamentally an epithelial 

 tube, only imperfectly segmented, which 

 contains not only direct reflex mechanisms 

 of the articulate type, but also a massive 

 continuous column of nerve cells and con- 

 necting fibers, the reticular formation, 

 which is a diffuse correlation center related 

 to all of the reflex arcs. In the head there 

 are special enlargements derived from this 

 (incompletely) segmented reticular forma- 

 tion, which make up the greater part of 

 the brain in a higher vertebrate. These are 

 the special correlation centers or the stipra- 

 segmental apparatuses of Adolf Meyer's 

 terminology. 



The entire vertebrate plan of nervous 

 system is totally different from that of any 

 member of the articulate series and, while 

 adapted to perform stereotyped reflexes 

 and instinctive modes of behavior, is also 

 capable of wholly different types of reac- 



