10 



SCIENCE 



[N. S. Vol. XXXI. No. 784 



tion based on the functional plasticity of 

 the reticular formation and its derivatives. 

 There is, of course, some measure of plas- 

 ticity in the behavior of arthropods, e. g., 

 some ability to learn by experience, and 

 they possess some tissue corresponding to 

 the reticular formation; but in the broad 

 view the distinctions just drawn are char- 

 acteristic of the two phyla. 



Without going into further detail, we 

 may, then, generalize that the higher in- 

 sects mark the culmination of the stereo- 

 typed or instinctive type of behavior, while 

 the primates represent the culmination of 

 the plastic, docile or rational type, and that 

 the structural basis of this plasticity of the 

 vertebrates is found in the relation of the 

 reticular formation of their nervous sys- 

 tems to the other elements of the neural 

 tube and especially in the suprasegmental 

 correlation centers derived therefrom. The 

 lower vertebrates are far inferior to the 

 higher insects in many respects— often per- 

 haps in the very ability to profit by experi- 

 ence of which we have been speaking; but 

 their physical organization is such as to 

 favor future differentiation in this direc- 

 tion, while that of the insects is such as to 

 forbid it. Thus it appears probable that 

 the dominance of the vertebrate type was 

 foreshadowed far back among the ancestral 

 crawling things in which no truly verte- 

 brate character was manifest, foreshadowed 

 merely by a structural type with different 

 latent potencies. 



The arthropod type of organization and 

 action system is rigidly stereotyped in the 

 race as well as in the individual; i. e., it 

 tends to be transmitted without modifica- 

 tion from generation to generation. Its 

 pattern can be changed only by natural 

 selection or some other agency which can 

 act through heredity. The more plastic 

 vertebrate type is not fixed completely at 

 birth by heredity, but its precise form is 



more largely acquired as individual experi- 

 ence advances. As intelligence plays a 

 progressively greater part in the behavior, 

 infancy will be prolonged to afford the 

 necessary opportunity for the plastic nerv- 

 ous system to be shaped in adaptation to 

 the individual needs of the animal. The 

 instruments of racial progress here are not 

 merely natural selection acting through 

 heredity, but also docility, social hered- 

 ity and organic selection, acting largely 

 through intelligent adaptation. 



In the vertebrates the amount of pre- 

 formed or inborn organization, both of 

 structure and of function, is in general 

 greater than in arthropods; but there is 

 superposed upon this rigidly predeter- 

 mined tissue in higher vertebrates the un- 

 specialized embryonic correlation tissue, 

 the details of whose organization are not 

 laid down in the hereditary pattern, but 

 are individually acquired during develop- 

 ment. The ultimate pattern which will be 

 assumed by this plastic tissue is largely 

 shaped by the exigencies of function dur- 

 ing the period of its immaturity and this 

 in turn rests upon the nature of the en- 

 vironmental factors. In short, the educa- 

 tional period is limited to the age during 

 which the epigenetic tissue, i. e., the corre- 

 lation centers whose form is not predeter- 

 mined in heredity, retains its plasticity 

 under environmental influence. 



Ultimately even the cerebral cortex ma- 

 tures and loses its power of reacting except 

 in fixed modes. Its unspecialized tissue 

 — originally a diffuse and equipotential 

 nervous meshwork — becomes differentiated 

 along definite lines and the fundamental 

 pattern becomes more or less rigid. The 

 docile period is past and, though the man 

 may continue to improve in the technique 

 of his performance, he can no longer do 

 creative work. He is apt to say, ' ' The dog 

 is too old to learn new tricks." Whether 



