134 evolution: the ages and tomorrow 



nect with one another and with overt responses in the higher 

 primates, and a one-trial learning becomes the common rule. 

 Nissen points out that with phyletic ascent the effective en- 

 vironment expands in space and time and increases the range 

 of behavior determinants, an expansion brought about by 

 the animal's ability to integrate and organize. The environ- 

 ment itself expands rapidly from generation to generation 

 with the advent of "culture," as we have seen in the review 

 of the evolution of human societies. 



Taking the summation of his review of the literature of 

 experimental psychology as a basis, Nissen presents a tenta- 

 tive estimate of the intelligence levels of the major subdivi- 

 sions of the animal kingdom. The higher protozoa rise to 

 levels comparable to the middle group of invertebrates. Ar- 

 thropods, the insect being one of the top representatives in 

 this phylum, rise sufficiently high in the scale to be well 

 above the level of lower vertebrates and overlap the low- 

 est mammals. The highest insect intelligence even matches 

 the lowest levels of man's own order, the primates. Verte- 

 brates other than the mammals rise to a level which is well 

 above that of the lowest mammals and overlap slightly the 

 lowest primate level. Mammals other than the primates 

 reach heights of intelligence comparable to the middle 

 group of monkeys. The highest monkeys and apes are defi- 

 nitely at the top of the scale and are surpassed only by mod- 

 ern man. Since we have no direct knowledge of the intelli- 

 gence level of the primitive forerunners of man, the highest 

 primate is placed below the lowest human on this scale. 

 From the indirect evidence available, however, anthropolo- 

 gists would not hesitate in placing the ancestors of man at 

 intelligence levels represented now by high monkeys and 

 apes, thus making it possible to establish a continuous ascent 

 on the behavior scale from protozoa to man. On the basis of 

 direct evidence obtained in recent years from comparative 

 studies of the behavior of higher primates and human chil- 



