LECTURES IN BIOLOGICAL SCIENCES 



descendants were not known. For instance, ancestral lineages 

 of the modern horse, rhinoceros, and the Biblical cony (Hyrax) 

 can all be traced back through a succession of fossil forms to 

 the Eocene period, fifty to sixty million years ago. At this pe- 

 riod, these lineages have converged to such an extent that the 

 forms representing them are much alike in size, shape, head 

 form, and tooth structure. Furthermore, the different repre- 

 sentatives of the same lineage which followed immediately after 

 each other in time were usually so much alike that they could 

 easily be visualized as having evolved through the accumula- 

 tion of many relatively small genetic differences. 



To be sure, many examples are known in which a new type 

 of animal or plant appears suddenly and seems to be completely 

 separate in respect to many large differences from any earlier 

 fossil form. To explain these apparent saltations Simpson as- 

 sumes that the fossil record contains many highly significant 

 gaps. Furthermore, both his evidence and logical arguments 

 suggest strongly that those conditions which would be most 

 likely to bring about the origin of a new major adaptive com- 

 plex and hence a new higher category would also be most likely 

 to produce gaps in the fossil record. Organisms which exist 

 as large populations in a stable environment have the greatest 

 chance of being preserved as fossils, but are the least likely 

 to give rise to new adaptive types. New departures in evolu- 

 tion are most likely to occur when a system of relatively small 

 populations, partly isolated from each other, is evolving in a 

 rapidly changing environment. This combination of conditions 

 is perhaps more unfavorable than any other for preserving such 

 forms as fossils. 



If, as Simpson believes, apparent saltations are produced by 

 a combination of rapid evolution plus unfavorable conditions 

 for fossilization, then those groups with the poorest fossil record 

 should have the largest number of apparent saltations, and the 

 improvement of our knowledge of the fossil record should pro- 

 gressively fill in the gaps. Recent progress in fossil discovery 

 has shown this to be true in a striking fashion. In particular, 

 the fossil history of man, which a generation ago had to be in- 

 terpreted on the basis of fragments which could almost have 

 been counted on the fingers of one's hands, now is illustrated 



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