CHAPTER XXXII 

 OTHER POSSIBLE GUIDING FACTORS 



Orthogenesis. — Geneticists as a rule feel that natural selection is 

 the only guiding factor needed to account for the adaptive features of 

 evolution. They are inclined to be skeptical about the definiteness of 

 the pathways of change that have been so strongly emphasized by 

 palaeontologists and others. T. H. Morgan at one time expressed the 

 view that most of the beautifully precise orthogenetic pedigrees, such 

 as that of the horse family for example, might be made up by selecting 

 only those fossil forms that fitted well into the progressive series and 

 ignoring those that did not fit so well. He showed that one could 

 make out a very pretty phylogeny of the spear heads displayed in 

 museums, in which each type leads to another type and all are capable 

 of arrangement into a few "orthogenetic" series. Since such a series 

 is merely an artificial arrangement without phylogenetic significance, 

 the same might also to some extent be true for the orthogenetic series 

 of the palaeontologist. Some geneticists are inclined to think that the 

 evolution of the horse family, for example, had been much less definite- 

 ly directed than it is commonly supposed to be. In each period there 

 existed numerous less progressive types and many aberrant species, as 

 well as those that seem to follow the more direct lines leading to the 

 modern horses. In other words, much of the definiteness of direction 

 is the result of focusing attention upon a few of the types that make a 

 good series and ignoring the ones that are not in line. This criticism 

 is possibly too iconoclastic. There surely is in the horse pedigree some 

 definiteness of trend amidst a flux of indefiniteness. Hence the prob- 

 lem of explaining this residual definiteness still remains. 



In recent years one of the staunch supporters of the reality of ortho- 

 genesis has been H. F. Osbom. His most striking material, perhaps, 

 consists of the fossil pedigrees of the Titanotheres, a group of extinct 

 ungulates somewhat resembling the modern rhinoceros. They ap- 

 peared relatively early in the evolution of mammals, and attained giant 

 proportions, great abundance, and wide distribution before becoming 

 extinct at the close of the Oligocene period. Though the titanotheres 

 started as small creatures about the size of terriers, they reached almost 

 the dimensions of elephants near the close of their career. One of 



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