CHAPTER SEVENTEEN 



Continuous versus 

 Discontinuous Variation 



Consideration of the living world makes nothing more obvious than 

 that variation is everywhere present. On the grandest scale, the Plant and 

 Animal Kingdoms are differentiated. Almost as obviously, the major types, 

 phyla, and classes, are differentiated within each kingdom. As one de- 

 scends the taxonomic hierarchy, variation is demonstrable within each 

 category, with the qualification that more refined methods of observation 

 may be needed to establish the fact at the lowest levels. Equally important 

 is the fact that the varying organisms are arranged into discontinuous clus- 

 ters, more specifically, into a hierarchy of discontinuous clusters. This is 

 the basic fact of taxonomy. 



THE BASES OF DISCONTINUOUS VARIABILITY 



Nonetheless, it is often maintained that the observed discontinuity in our 

 present-day flora and fauna is illusory because it depends upon the ex- 

 tinction of intermediates which have actually existed in the past. Accord- 

 ing to this viewpoint, if representatives of every species that has ever 

 existed could be arranged in order from the most primitive to the most 

 specialized, an almost imperceptible transition would be observed, with 

 abrupt discontinuities corresponding only to single gene differences. In 

 other words, the entire living world would show no discontinuities greater 

 than those which actually characterize single species. 



Admittedly, no such assemblage could ever be possible because of the 

 immense amount of extinction which has marked the history of life upon 

 this earth. Nor is it possible to assemble a sufficiently complete fossil series 

 to simulate the series suggested, for the fossil record is very incomplete. 

 But the neo-Darwinian viewpoint is favorable to the possibility that an 

 approximately continuous series may have existed, though never at a 

 single time level. The alternative is, of course, that the larger steps in 

 evolution may have been achieved through systemic mutations, or through 

 some other type of macroevolutionary change. If this is correct, then at 



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