624 



SCIENCE 



[N. S. Vol. LI. No. 1330 



The insects comprise the highest inverte- 

 brates and as a group they are remarkably 

 efficient animals; but their extremely diverse 

 specialization is spread out on a rather low 

 plane and the behavior of each individual 

 member of the group is tolerably rigidly 

 limited to a narrow range of instinctive acts 

 with small capacity for individual modifi- 

 ability. The extreme plasticity of the group 

 of ants as a whole, so graphically portrayed 

 by Wheeler,^ has been biologically determined 

 through natxiral selection or otherwise by the 

 adaptation of each of the diverse species and 

 castes for a very special mode of life which 

 must be followed through, with no consider- 

 able deviation. This is in sharp contrast 

 with the plasticity of the higher mammals 

 which rests rather on capacity for modifi- 

 ability, docility and intelligent adaptation to 

 new conditions of each individual animal. 



Similarly, within the vertebrate phylum we 

 find divergent modifications of the primary 

 tubular pattern of the central 'nervous system, 

 each of which, as soon as matured and 

 stabilized in the inherited organization, favors 

 subsequent differentiation in certain direc- 

 tions and precludes it in others, for this 

 differentiation is irreversible. 



The teleostean type of forebrain is quite 

 unlike anything else in nature. It probably 

 was early forecast in primitive ganoids with 

 brains like those of the modem sturgeons, 

 where there is no evagination of the cerebral 

 hemispheres but instead local thickenings in 

 the unevaginated walls of the rostral portion 

 of the neural tube. Once this method of 

 differentiation was established, there is no 

 evidence that it ever gave rise to the type 

 represented by Amphibia and all Amniota 

 with hollow hemispheres. The teleosts, like 

 the insects, are very efficient organisms and 

 in the aggregate they adjust to a wide variety 

 of conditions, but they are differentiated on a 

 relatively low plane, the structiiral and be- 

 havior patterns of each species are rigid and 

 narrowly circumscribed, and the group has 

 given rise to none of the higher types. 



In the primitive reptilian stock there was 



» "Aits," New York, 1913. 



another divergence in pattern of forebrain 

 evolution. One type developed masive basal 

 nuclei in the cerebral hemispheres, as in 

 modern saurians. This line of differentia- 

 tion advanced to ciilminate in modern birds 

 with basal nuclei (striatum complex) more 

 massive than in any other animals and with 

 very insignificant cerebral cortex. In corre- 

 lation with this, the birds on the behavior 

 side present the culmination of instinct, 

 with intelligence of low order. A second 

 reptilian type, starting with brain forms more 

 like those of the modem turtles, followed a 

 different line of differentiation and led up to 

 the mammalian type with wide lateral ven- 

 tricles and extensive superficial cerebral 

 cortex. This type seems better adapted to 

 develop into an adequate organ of intelligent 

 behavior, and in this direction it appears not 

 yet to have reached its limit. 



In speaking of the influence of the arboreal 

 habitat upon the evolutionary history of 

 Primates, F. Wood Jones,° draws an interest- 

 ing contrast between this phylum and the 

 arboreal marsupials (Metatheria), in the fol- 

 lowing passage: 



These arboreal Metatheriaas have had all the 

 educational advantages of a thoroughly arboreal 

 life; nothing that we have pictured has failed to 

 exert its influences upon them, and yet it is ob- 

 vious that the advantage that they have taken of 

 it has been slight. There are metatherian conver- 

 gent mimics of Carnivora, Eodentia, Insectivora, 

 ajid of most other Eutherian orders, but there is 

 no metatherian convergent mimic of the eutherian 

 Primates. It would not be unnatural, therefore, to 

 assume that the full advantage could not be 

 grasped by the metatherian animals, since the 

 ground-plan of their brain would not permit it. 



The argument continues that the absence 

 of the corpus callostmi in Metatheria gives 

 the clew to this orthogenetic limitation. 



From these and innumerable similar in- 

 stances familiar to every comparative anato- 

 mist it may be argued that the process of 

 differentiation, so far as this represents an 

 irreversible proees3, is itself a natural cause 

 of limitation of the future course of evolution 



9 ' ' Arboreal Man, ' ' London, 1916. 



