THE PRINCIPLES OF BIOLOGY. 
47 
orifices give it an external two-sidedness at the same time 
that they establish a difference between the two ends, yet its 
two-sidedness is not strongly marked. . . . On calling to 
mind the habits of the creatures . . . it will be seen that 
they explain these forms. The incidence of forces is the same 
all round the Earth-worm as it burrows through the compact 
ground.” 
The forms of the Mollusca are next surveyed from the 
same point of view, and then the forms of the Vertebrates 
are considered. Mr. Spencer here shows that, as the front 
and hinder parts, and also the upper and lower parts of the 
higher animals are differently formed, so the conditions to 
which these unlike parts are subject are different. Moreover, 
such unlike parts are found to be more unlike in proportion 
to the degree of unlikeness in the conditions affecting them. 
And an illustration fp. 188) is found in the Pleuronectidse— 
“ the order of distorted flat fishes to which the Sole and the 
Flounder belong. On the hypothesis of evolution, we must 
conclude that fishes of this order have arisen from an 
ordinary bilaterally symmetrical type of fish, which, feeding 
at the bottom of the sea, gained some advantage by placing 
itself with one of its sides downwards, instead of maintaining 
the vertical attitude. Besides the general reason, there are 
specific reasons for concluding this. In the first place, the 
young Sole or Flounder is bilaterally symmetrical—has its 
eyes on opposite sides of its head, and swims in the usual 
way. In the second place, the metamorphosis which produces 
the unsymmetrical structure sometimes does not take place— 
there are abnormal Flounders that swim vertically, like other 
fishes. In the third place, the transition from the symmetrical 
structure to the unsymmetrical structure may be traced. 
Almost incredible though it seems, one of the eyes is trans¬ 
ferred from the under side of the head to the upper side(p. 189 ) 
. . . . besides this divergence from bilateral symmetry 
involved by the presence of both eyes upon the upper side, 
there is a further divergence from bilateral symmetry involved 
bv the differentiation of the two sides.” 
In a last quotation from a summary of the evidence (p. 189), 
we read:—“ Thus little as there seems in common between 
the shapes of plants and the shapes of animals, we yet find 
on analysis that the same general truths are displayed by 
both. The one ultimate principle, that in any organism equal 
amounts of growth take place in those directions in which the 
incident forces are equal, serves as a key to the phenomena of 
morphological differentiation. By it we are furnished with 
interpretations of those likenesses and unlikenesses of parts 
