Chap. XIII. 
SUMMAEY. 
457 
importance, or, as in rudimentary organs, of no import¬ 
ance ; the wide opposition in yalue between analogical 
or adaptive characters, and characters of true affinity; 
and other such rulesall naturally follow on the view 
of the common parentage of those forms which are con¬ 
sidered by naturalists as allied, together with their modi¬ 
fication through natural selection, with its contingencies 
of extinction and divergence of character. In consider¬ 
ing this view of classification, it should be borne in 
mind that the element of descent has been universally 
used in ranking together the sexes, ages, and acknow¬ 
ledged varieties of the same species, however different 
they may be in structure. If we extend the use of 
this element of descent,—the only certainly known 
cause of similarity in organic beings,—we shall under¬ 
stand what is meant by the natural system: it is 
genealogical in its attempted arrangement, with the 
grades of acquired difference marked by the terms 
varieties, species, genera, families, orders, and classes. 
On this same view of descent with modification, all 
the great facts in Morphology become intelligible,—^ 
whether we look to the same pattern displayed in the 
homologous organs, to whatever purpose applied, of the 
different species of a class; or to the homologous parts 
constructed on the same pattern in each individual 
animal and plant. 
On the principle of successive slight variations, not 
necessarily or generally supervening at a very early 
period of life, and being inherited at a corresponding 
period, we can understand the great leading facts in 
Embryology ; namely, the resemblance in an indivi¬ 
dual embryo of the homologous parts, which when 
matured will become widely different from each other 
in structure and function; and the resemblance in 
different species of a class of the homologous parts or 
X 
