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,t “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. 
tFrom A. F. Shull, Principles of Animal Biology (copyright 1920). Used by 
special permission of the publishers, The McGraw-Hill Book Company. 
