, though the number and proportion of parts may{8}
more or less differ. Again, the butterfly and the shrimp, different as they
are in appearance and mode of life, are yet constructed on the same common
plan, of which they constitute diverging manifestations. No _a priori_
reason is conceivable why such similarities should be necessary, but they
are readily explicable on the assumption of a genetic relationship and
affinity between the animals in question, assuming, that is, that they are
the modified descendants of some ancient form--their common ancestor.

That remarkable series of changes which animals undergo before they attain
their adult condition, which is called their process of development, and
during which they more or less closely resemble other animals during the
early stages of the same process, has also great light thrown on it from
the same source. The question as to the singularly complex resemblances
borne by every adult animal and plant to a certain number of other animals
and plants--resemblances by means of which the adopted zoological and
botanical systems of classification have been possible--finds its solution
in a similar manner, classification becoming the expression of a
genealogical relationship. Finally, by this theory--and as yet by this
alone--can any explanation be given of that extraordinary phenomenon which
is metaphorically termed _mimicry_. Mimicry is a close and striking, yet
superficial resemblance borne by some animal or plant to some other,
perhaps very different, animal or plant. The "walking leaf" (an insect
belonging to the grasshopper and cricket order) is a well-known and
conspicuous instance of the assumption by an animal of the appearance of a
vegetable structure (see illustration on p. 35); and the bee, fly, and
spider orchids are familiar examples of a converse resemblance. Birds,
butterflies, reptiles, and even fish, seem to bear in certain instances a
similarly striking resemblance to other birds, butterflies, reptiles, and
fish, of altogether distinct kinds. The explanation of this matter which
"Natural Selection" offers, as to animals, is that certain varieties of {9}
one kind have found exemption from persecution in consequence of an
accidental resemblance which such varieties have exhibited to animals of
another kind, or to plants; and that they were thus preserved, and the
degree of resemblance was continually augmented in their descendants. As to
plants, the explanati