CHAPTER TWENTY-ONE 



Retrospect and Prospect 



In reteospect, we have reviewed the main outlines of the concept of 

 organic evolution as modern scientists have developed it, as well as the 

 reasons which have led them to this viewpoint. We have reviewed the 

 fossil record, and seen that this is the most impressive evidence of evolu- 

 tion, in spite of its grave deficiencies. Finally, we have reviewed the mod- 

 ern attempts to analyze the genetic, cytological, ecological, geographical, 

 and other causative factors in evolution, and we have seen that these com- 

 plement one another to form a synthetic whole. 



The dominant theme of recent work in evolution has been the neo- 

 Darwinian theory, according to which the basic phenomenon of evolution 

 is the slow accumulation of small mutations, the screening out of combina- 

 tions of these by ecological factors which comprise natural selection to 

 form subspecies, and finally the formation of good species by the same 

 processes, aided by isolating mechanisms which prevent the subspecies 

 ( incipient species ) from merging with the general population from which 

 it came. 



A second theme, urged by Goldschmidt and others, has emphasized the 

 changes of chromosomal architecture which commonly distinguish related 

 species, and which may be the basis for systemic mutations which pro- 

 foundly change the organism. By this means, new species could be formed 

 in one or a few steps, but they would then immediately be subject to the 

 test of natural selection, and to subspeciation by neo-Darwinian evolution. 

 Polyploidy, especially allopolyploidy, might also be properly included 

 here, but both schools agree that this is a special, though important, phe- 

 nomenon from which generalization is not justified. 



The clash between these viewpoints has been acrimonious at times, yet 

 they may prove to be supplementary rather than mutually exclusive. They 

 were originally based upon different theories of the gene, morphological 

 and atomistic for the neo-Darwinians, physiological and integrated for 

 Goldschmidt. The physiological consequences of structure and the struc- 

 tural consequences of physiology were not clear. Today, much evidence 

 indicates that patterns within nucleic acid macromolecules are critical for 

 the gene. In terms of chromosomes, the gene may be a more or less broad 

 field, with a point focus comparable to a physical center of gravity. These 

 functional areas may vary in size, and they may overlap broadly, so long 



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