28 FRIENDS WORTH KNOWING. 
many species. Such changes may have happened any- 
where, again and again, and thus the two halves of a com- 
munity been divided. In succeeding centuries the mem- 
bers of the parted sections may have diverged in their de- 
AN ALIEN IN THE CELLAR. 
velopment, until on this side of a mountain range, or desert, 
or sea, we now find one set of species and on that side an- 
other set, which belong to the same genera, and may in 
some cases be proved, as well as surmised, to have had an 
identical origin. 
