100 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Arctic zone has, therefore, been in past times capable of supporting 

 almost all of the forms of life of our temperate regions; and we must 

 take account of this condition of things whenever we have to specu- 

 late on the possible migration of organisms between the old and new 

 continents." 



"Many of the facts of distribution," says Shull,' "are capable of 

 interpretation by the assumption that evolution has operated with the 

 other factors. If each kind of animal has arisen from a pre-existing 

 kind, then each group of related animals must have had an ancestral 

 form, and if the component parts of the groups are widespread the 

 range of the ancestral form may be considered to be the center of 

 dispersal of the group. The facts of distribution can apparently be 

 interpreted only on this basis. 



"Accepting evolution, along with the other factors which can be 

 recognized, the method of distribution is generally conceived to be as 

 follows. The ancestral form tends to spread in all directions. In 

 some directions it is limited by unfavourable conditions either through- 

 out its life or for some time. In other directions it extends its range. 

 Anywhere within its range new types of individuals may arise through 

 the process of evolution. These new types may be fitted to occupy 

 new regions, and if they are formed near the limits of the range they 

 may find opportunity to spread into areas which are inaccessible to 

 the unaltered members of the species. Thus may arise recognizably 

 distinct forms coincident in range with certain environmental condi- 

 tions. If particular forms, or the individuals of a single form, are 

 accidentally (or possibly by sporadic migration) transferred across 

 barriers the distribution of the group becomes discontinuous. If 

 these processes have been going on for a long time, that is, if the 

 common ancestors of a group of forms existed long ago, the range may 

 have had time to become very extensive, or its discontinuity very 

 marked. If, contrariwise, the ancestors were comparatively recent, 

 the range is likely to be much smaller. For this reason, groups that 

 have diverged far enough to have attained the rank of families are on 

 the whole more widespread than those so nearly allied as to be con- 

 sidered genera. Should the environment become altered within a 

 given range, the occupying form might be driven from it or destroyed . 



^ From A. F. Shull, Principles of Animal Biology (copyright 1920). Used by 

 special permission of the pubhshers, The McGraw-Hill Book Company. 



