112 THE BRAIN OF THE TIGER SALAIVIANDER 



general morphogenic agencies which antedate the speciaHzed features 

 of ontogenetic differentiation. The old controversies about preforma- 

 tion versus epigenesis are outmoded and may well be ignored, for 

 both factors are present in all development. It can no longer be 

 claimed that "the basis of developmental pattern is an inherent 

 property of protoplasm and therefore continuously present and in- 

 dependent of external conditions" (Child, p. 6), for at no period in 

 the life of any organism is it independent of its environment. The 

 vital process is fundamentally an interaction between the proto- 

 plasmic organization and its surroundings— respiration, nutrition, 

 and all the rest. But when a given heritable pattern of internal or- 

 ganization has once been established, then the several components of 

 this organization may acquire a large measure of autonomy in both 

 further development and adult function. This specialization has ob- 

 viously taken place; and when, in the course of development of 

 vertebrate neuromuscular organs, they are approaching functional 

 capacity, the patterns of this performance, though not independent 

 of internal and external environment, clearly are not directly deter- 

 mined by any specific influences acting upon them from receptor 

 organs. In this restricted sense these patterns are initiated intrinsi- 

 cally, and their performance is autonomous. But throughout develop- 

 ment there is a complicated interaction of the inherited factors with 

 one another, and this intrinsic interplay of moiphogenic agencies is 

 an important feature of developmental mechanics. 



The widely current belief that heritable variations sometimes occur 

 in progressive series set in a definite direction rather than always in 

 accordance with the probability curve of chance deviations around 

 fixed unit characters has been strongly supported by many independ- 

 ent lines of evidence. It has been pointed out that the progressive 

 senescence of tissue in both ontogenetic and phylogenetic series in- 

 volves a fixation or stabilization of originally undifferentiated plastic 

 tissue into permanent structural patterns. So far as this differ- 

 entiation is heritable and irreversible, the future course of evolu- 

 tion is thereby intrinsically determined, for variations will be dis- 

 tributed around the new pattern as a mode in accordance with a dif- 

 ferent frequency curve than would be shown if the inherited struc- 

 tural pattern were different. The process of differentiation is there- 

 fore itself a natural cause of limitation of the future course of evolu- 

 tion within boundaries set by the efficient working of the established 

 pattern. The nervous systems of arthropods, teleosts, reptiles, birds, 



