In considering 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, dimorphic forms, and acknowledged varieties of
the same species, however much they may differ from each other in
structure. If we extend the use of this element of descent--the one
certainly known cause of similarity in organic beings--we shall understand
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, most of the great facts in
Morphology become intelligible--whether we look to the same pattern
displayed by the different species of the same class in their homologous
organs, to whatever purpose applied, or to the serial and lateral
homologies 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 leading facts in
embryology; namely, the close resemblance in the individual embryo of the
parts which are homologous, and which when matured become widely different
in structure and function; and the resemblance of the homologous parts or
organs in allied though distinct species, though fitted in the adult state
for habits as different as is possible.
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