of fishes performing the same office as the lungs of
the mammalia; and these different organs sometimes exist, at different
periods, according to the degree of development, in the same animal.
Thus, the tadpole, so long as it continues to be a fish, breathes by
gills, which disappear and give place to lungs when it becomes a frog.
Similar transformations of the insect tribe are familiar to all.
Imperfect or rudimentary organs are found in certain animals, as the
mammae of a man; a particular organ being here developed to a certain
extent, though it is not needed; but being developed a little further,
it becomes useful in the next set of animals in the scale. The same
peculiarity is found among plants; the skilful gardener being able
actually to develope these rudimentary organs by supplying the requisite
conditions, and thus, as it were, to raise the plant one step in the
scale.

"We have yet to advert to the most interesting class of facts
connected with the laws of organic development. It is only in
recent times that physiologists have observed that each animal
passes, in the course of its germinal history, through a series of
changes resembling the _permanent forms_ of the various orders of
animals inferior to it in the scale. Thus, for instance, an insect,
standing at the head of the articulated animals, is, in the larva
state, a true annelid, or worm, the annelida being the lowest in
the same class. The embryo of a crab resembles the perfect animal
of the inferior order myriapoda, and passes through all the forms
of transition which characterize the intermediate tribes of
crustacea. The frog, for some time after its birth, is a fish with
external gills and other organs, fitting it for an aquatic life,
all of which are changed as it advances to maturity, and becomes a
land animal. The mammifer only passes through still more stages,
according to its higher place in the scale. Nor is man himself
exempt from this law. His first form is that which is permanent in
the animalcule. His organization gradually passes through
conditions generally resembling a fish, a reptile, a bird, and the
lower mammalia, before it attains its specific maturity. At one of
the last stages of his foetal career, he exhibits an
intermaxillary bone, which is characteristic of the perfect ape;
this is suppressed, and he may then be s