y^The Consequences 

 of Evolution 



EVERY MUTATION or other heritable 

 change, if it becomes established in a species, 

 either by natural or artificial selection, repre- 

 sents a definite evolutionary step, and in this 

 sense evolution is directly demonstrable by 

 experiment. But the conclusion that all exist- 

 ing species have originated by similar proc- 

 esses occurring in past ages cannot — in the 

 nature of the case — be subjected to direct 

 proof. In accordance with the scientific prin- 

 ciple of the uniformity of nature, however, 

 evolution represents a logically predictable 

 consequence of known forces and processes. 

 Geology and physics provide good evidence 

 (p. 559) that organisms have lived on earth 

 for more than three billion years, while the 

 environment, aside from large-scale fluctua- 

 tions in local conditions, has not undergone 

 great change. It may be assumed, therefore, 

 that variation and selection have operated 

 during these past ages at much the same rate 

 as they do today, and this rate, operating 

 through this expanse of time, seems ample 

 to account for the evolution of the great 



552 



diversity of existing species. Moreover, evo- 

 lution has left indelible imprints upon many 

 structural and functional characteristics in 

 every organism, and the crust of the earth 

 contains the fossilized remnants of man}' 

 kinds of pre-existing organisms. Thus an 

 accumulation of evidence from many sources 

 impels the scientific mind to accept the evo- 

 lutionary viewpoint. Organic evolution does 

 not necessarily demand that all existing or- 

 ganisms have arisen from a single primitive 

 ancestral form, but the bulk of evidence in- 

 dicates that early primordial life was very 

 simple and did not consist of any great 

 diversity of forms. 



Evolution has left its mark upon every 

 phase of biology, as has been noted in many 

 places throughout the earlier chapters. In 

 summary, therefore, the consequences of evo- 

 lution will be considered very brieflv, under 

 the topics of: classification, comparative 

 anatomy, embryology, biochemistry, and 

 paleontology (a systematic comparative study 

 of fossils). 



