BASIS OF PSYCHICAL-SOCIAL INDIVIDUALITY 611 



from the sensory stimuli received from the environment depends, therefore, 

 not only on the character of the sense organs, but also on the central nervous 

 system. 



In correspondence with this evolution of the nervous system there seems 

 to take place also an increasing complexity in the production and action of 

 hormones. The number of hormones which have so far been demonstrated in 

 invertebrates is relatively small and their presence has been noted principally 

 in the higher types, particularly in the arthropods. To mention some of the 

 hormones and their actions which have been studied best : The experiments 

 of Roller and Perkins, and others, have shown the existence of a hormone 

 which is involved in the movement of pigment in the chromatophores of 

 crustaceans; it is produced in the eyestalk in the sinus gland. Analogous 

 hormones may affect also the chromatophores of various classes of verte- 

 brates (G. H. Parker, F. B. Sumner). Kopec and others found that in 

 Lepidoptera the supraesophageal ganglion controls pupation. In addition in 

 Hemiptera and Orthoptera molting and pupation are induced by hormones 

 originating in nervous ganglia. According to Wigglesworth a hormone given 

 off by the corpus allatum inhibits metamorphosis. In Drosophila a hormone 

 responsible for pupation is given off by the larval ring gland which is 

 situated between the two hemispheres of the larval brain (Hadorn), and 

 Bodenstein has moreover made it probable that this gland induces also the 

 differentiation of organs into the imaginal state. The last mentioned effect may 

 however be an indirect one and as in the case of other hormones already 

 discussed, the changes induced by the ringgland do not depend solely on 

 the nature of the hormone, but on a balance between hormone and the state 

 of the recipient tissue. Various other hormones probably exist in inverte- 

 brates ; there are in particular indications in crustaceans and also in other 

 invertebrates that hormones may affect the development of secondary sex 

 characteristics ; furthermore the socalled gene hormones might be mentioned 

 in this connection. As in the case of vertebrates so also in invertebrates no 

 strict species specificity of hormones seems to exist and there are even 

 indications that invertebrate hormones may perhaps affect vertebrate organs 

 and conversely certain vertebrate hormones may affect invertebrate tissues. 



The relative scarcity of the hormones so far discovered in invertebrates as- 

 well as their distribution in different classes suggests that an evolution com- 

 parable to the evolution in the central nervous system may have taken place 

 also in the case of hormones; this would be in accordance with the simpler 

 structure, the less developed differentiation, integration and coordination, the 

 more limited organ functions in these more rudimentary animals as compared 

 with the conditions found in vertebrates. However, there are no exact data 

 available which would make possible at the present time a definite com- 

 parison of the number of hormones present in a tissue unit in vertebrates 

 and invertebrates, and it will be a task for future research to trace the 

 phylogenetic evolution of hormonal regulations. 



In addition to these central mechanisms there are the peripheral receptors, 

 the sense organs, which in response to physico-chemical factors emanating 



