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ponding sides of the human body ; vertical, represented 
by the vertebrae of the axolotl. 
“ Parts which are homologous tend to vary in the same manner ; 
and this is what might have been expected, for such parts are identical 
in form and structure during an early period of embryonic develop 
ment, and are exposed in the egg or womb to similar conditions.” 
—(The Variation, vol. ii., p. 322.) 
That such sympathetic modifications take place by 
reason of an inherent principle, will be seen from the 
following passage, in which Sir James Paget, speaking 
of symmetrical disease, says :— 
“A certain morbid change of structure on one side of the body 
is repeated in the exactly corresponding part of the other side.” 
He then quotes and figures a diseased lion’s pelvis, from 
the College of Surgeons’ Museum, and says of it:— 
“ Multiform as the pattern is in which the new bone, the product of 
some disease comparable with human rheumatism, is deposited— 
a pattern more complex and irregular than the spots upon a map— 
there is not one spot or line on one side which is not represented 
as exactly as it would be in a mirror on the other. The likeness 
is more than daguerreotype exactness.”—(.Lectures on Surgical 
Pathology. 1853. v °l- *■» PP* 18-22.) 
It is obvious that this principle of correlated variation 
is a most important factor of modification, because it 
extends the influence of modification beyond the part 
which is immediately affected ; and as the different parts 
are associated on different principles of correlation, a very 
considerable change may be made in different directions 
throughout the whole structure. 
We have now to consider in what relation this tendency 
to modification inherent in an organism composed of co 
ordinated parts and produced by a process of development, 
stands to any causes of modification which may act upon 
it from without; and especially what is the relation 
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