DISCONTINUITY OF SPECIES. 35 
subject to them on the whole form a discontinuous series. * * * Tempera- 
ture, altitude, depth of water, salinity, in fact most of the elements which make 
up the physical environment are continuous in their gradations, while as a rule 
the forms of life are discontinuous. Besides this, forms which are apparently 
identical live under conditions which are apparently very different, while species, 
which though closely allied are constantly distinct, are found under conditions 
- which are apparently the same. 
He suggests that the explanation of this lack of correspondence 
must be sought in the organic group, and not in its environment; and 
that in the study of variation is the chief hope though even that may 
fail (p. 17). 
I entirely agree with Mr. Bateson in regard to the importance of 
variation and of the factors in the species that control variation; and 
prominent among these factors I find either the power of free com- 
munication and intergeneration or the lack of this power. Whena 
species possessing very limited powers for migrating and very rare 
opportunities for transportation is surrounded by a mass of species 
having the usual powers and opportunities, the natural result is that 
many colonies from the species having very limited powers become 
completely isolated from each other and from the original stock, 
while the surrounding species of plants and animals are not isolated 
from the groups of the same species in other places. A further result 
is that the isolated colonies become divergent, while the species hav- 
ing powers that prevent isolation remain the same as in the original 
home. The isolated groups being prevented from crossing with each 
other, there is nothing to prevent each group from establishing its 
own special methods of dealing with the environment, and thus sub- 
jecting itself to special forms of selection, though dealing with the 
same environment. No better examples can be found of the diver- 
gence of isolated groups while exposed to the same environment than 
those presented by the arboreal snails of the Hawaiian Islands and of 
some other regions. 
The probabilities are completely reversed in the case of a species 
possessing unusual powers for migrating, or extraordinary opportu- 
nities for transportation. For it often happens that groups of such a 
species, occupying districts very unlike, not only in climate but in the 
prevalent species of plants and animals, will maintain free communi- 
cation and intergeneration with each other, and thus be kept to 
essentially one type. Good examples of unity of type maintained 
notwithstanding long exposure to diverse conditions are found in the 
case of certain species of birds and insects possessing great powers of 
flight. 
