180 
THE PRINCIPLES OF BIOLOGY. 
may be, there is a clear distinction between the parts 
exposed to view and the parts enclosed by them. The plant 
“ presents a contrast between its peripheral substance and 
its central substance.” This difference may, indeed, be but 
slight, but it is inevitable. In whatever way the organism is 
viewed this fact appears first and prominently. The differ¬ 
ences in the various species are naturally varied and numerous, 
but however unlike some of them may be they all agree in 
this one important particular, “a strong distinction between 
the parts in contact with the environment and the parts not 
in contact with the environment.” It is to this fact, simple 
as it may seem, that differentiation of function is due. 
In the consideration of the “ differentiations among the outer 
tissues of plants,” Mr. Spencer instances two forms of plant 
life, the Protococcus and I olvox Glohator , two free forms. The 
Volvox is a complete sphere, and, consequently, no part of its 
outer surface is different to another, and forces act equally 
upon the whole of its exterior substance. This is anticipated 
by the hypothesis < ‘If differentiations are occasioned by 
differences in the incidence of forces, then there will be no 
such differentiations where there are no such differences.” On 
the other hand, considerable differences are noticeable where one 
end of any organism is attached to some object; then the differ¬ 
entiation is very great, because the difference in the incidence 
of the forces has been great also. 
In the case of the Phanerogams the environments of the 
fixed part and the free part of the plant are entirely different. 
The root is buried in the ground ; that is, its environment is 
the earth. The environment of the stem, branches, and 
leaves is the air. 
It is needless to point out the differences between the 
roots of a tree and its stem and branches; they are obvious; 
but the question is, How came these differences to exist ? If 
we take a cutting from a tree and place a portion of it in the 
earth, that part, if all conditions are favourable, speedily 
develops rootlets and root fibres ; in fact, part of a stem has 
assumed the characteristics and the functions of a root. And to 
what is this change due ? To the altered environment, and to 
the incidence of different forces. But, as Mr. Spencer says, “ the 
most conclusive evidence is furnished by the actual substitutions 
of surface-structures and functions that occur in aerial organs, 
which have taken to growing permanently under ground, and 
in underground organs which have taken to growing 
permanently in the air. On the one hand, there is the 
Rhizoma, exemplified by ginger.On the other 
hand, there are the aerial roots of certain orchids.” We 
