CHAPTER 5 



Correlation 



The most significant fact about organic growth, as described in the pre- 

 ceding chapters, is that it is a process under definite control and thus leads 

 to the development of bodies of definite form and size. This control is 

 shown in the character of the growth cycle itself which, as we have seen, 

 marches forward in an orderly fashion to the attainment of a specific size. 

 It is shown even better in the distribution of this growth during develop- 

 ment. If growth were equal in all parts and directions, organisms would 

 be spherical. The remarkable variety of forms that living things display 

 and that constitutes one of their important differences from most lifeless 

 objects is due to the fact that the amount of growth in one region is dif- 

 ferent from that in another and that its rate in the various dimensions of 

 a structure is unequal. These differences are not random ones, for if they 

 were, a jumble of fantastic forms would result; they arise in an orderly 

 sequence and progress in a regular fashion until a specific organic struc- 

 ture is developed. Something evidently guides the growth and differenti- 

 ation of the organic mechanism. Occasionally this control is seriously 

 disturbed and in such cases abnormal growths and monstrosities of various 

 kinds appear (p. 275), but in the great majority of cases orderly develop- 

 ment and the production of specifically formed structures take place. It is 

 clear that in some fashion the parts of an organism are so related to each 

 other that a change in one affects the rest and that the whole is thus inte- 

 grated into an organized system. 



All the phenomena of development which are to be discussed— polarity, 

 symmetry, differentiation, regeneration, and the rest— are simply different 

 aspects of this developmental relatedness, and the various factors con- 

 cerned are those which have been found to affect it in one way or another. 

 The fundamental causes of this integrated development are yet unknown. 

 They are often attributed to correlation, a term which, because it is in 

 most cases merely a name for our ignorance, has with many students of 

 morphogenesis fallen into disrepute. Nevertheless correlation is a fact, 

 explain it how we will, and no one can approach a study of the phe- 

 nomena of morphogenesis without recognizing this. Therefore at the be- 



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