PATTERNS OF DEVELOPMENTAL PROGRESS 727 



for the skull of the chimpanzee (B) and the baboon (C), as compared with 

 the human skull (A), are shown. The transformations in B and C are 

 of the same order, differing only in degree. That alterations of gradient 

 patterns are concerned in the differences in form indicated in the figures 

 seems evident. 



If gradient characteristics, length, steepness, etc., are concerned in 

 determining localization of particular differentiations, it is evident that 

 genetically determined changes in these characteristics may alter locali- 

 zations of organ systems and organs relatively to others. The kind of 

 differentiation that takes place at any particular gradient-level may be 

 altered by other genetically determined changes in constitution of the 

 protoplasmic substrate. If gradient patterns are essential factors of de- 

 velopment, it is to be expected that genetic changes altering gradient 

 patterns may often involve features apparently only slightly related or 

 quite unrelated. The genetic change may alter whole gradient systems, 

 either the general systems of the whole body or the systems of particular 

 organs, and so alter localization, differential growth, and differentiation 

 of many parts. The possibility that a single mutation may determine 

 many such alterations can scarcely be denied. In so far as genetic changes 

 permitting survival and reproduction have effects of this sort on the more 

 general gradient patterns, organisms or organ systems evolve more or 

 less as wholes with orderly relations of their parts. 



