168 THE PROBLEMS OF EVOLUTION 



materials and potentialities. Bones are distinct 

 entities, intricately associated with the surround- 

 ing tissues, but they occasionally develop in situ- 

 ations where their appearance is not usual and so 

 demonstrate the truth of this assertion. Sesamoid 

 bones are a fairly common case in point. Less 

 normal are cases of reversion, such as the develop- 

 ment of one of the normally absent toes of the 

 horse, 22 and abnormal development of joints as a 

 result of fracture, in place of normal healing. 

 Everything in the organism, in short, is a resultant 

 of something which has preceded it, both onto- 

 genetically and phylogenetically, whether or not 

 its history in the two fields corresponds in detail. 

 The whole idea is nothing more than the very 

 logical deduction from past knowledge of evolution 

 which has gained eager attention within the past 

 decade under the title of emergent evolution. This 

 doctrine, advanced as new in recent years but in 

 reality only a clear expression of old truths, is 

 based on the conclusion that the attainment of 

 one step in evolution makes possible entirely new 

 steps. To the extent that emergent evolution con- 

 fines itself to this assertion it is not only sound 

 but simple. When we add the statement that later 

 developments are not an expression merely of the 

 properties of their components in a simpler dis- 



22 Boas, J. E. v., Zool. Jahrb. Alt. f. Anat. u. Ont. der Tiere, Vol. XL, pp. 

 49-104, 1917; Vol. LI (2), pp. 313-322, 1929. 



