THEORIES AND MECHANISM OF EVOLUTION 



653 



Fig. 26-2. A series of pelts of the striped skunk (/Mephitis). These are all of the same species, yet they show wide 



variation in regard to the white-stripe pattern. 



these widely different conditions. Thus 

 more than one species might originate si- 

 multaneously or at different times from one 

 ancestral form. As ages pass, tlie differences 

 between these may become great enough 

 to classify them as separate genera. This 

 continues through families, orders, classes, 

 and finally phyla, thus accounting for the 

 entire animal kingdom. Obviously, all that 

 is necessary to establish evolution is to 

 explain how a species originates and Dar- 

 win felt, as we do today, that natural selec- 

 tion was the answer. 



Modern interpretation 



Although the theory of natural selection 



has withstood the attacks of several o;enera- 

 tions of scientists, it must be interpreted in 

 the light of information that has come to 

 us since Darwin's time. The science of 

 genetics has made it possible to interpret 

 one very important aspect of natural selec- 

 tion for which Darsvin had no information, 

 namely, variations. Remember that varia- 

 tions provide the raw materials on which 

 natural selection operates. Darwin was well 

 aware that variations were essential to evo- 

 lution but he could not explain how they 

 occurred. We know now that they result 

 from gene changes or mutations. Any vari- 

 ations due to environmental changes during 

 the life of the individual, in the embryo or 



