into a minute, single, and very simple eye-spot.  In this last and complete state, cirripedes may be considered as either more highly or more lowly organised than they were in the larval condition.  But in some genera the larvae become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males:  and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction.

We are so much accustomed to see differences in structure between the embryo and the adult, and likewise a close similarity in the embryos of widely different animals within the same class, that we might be led to look at these facts as necessarily contingent in some manner on growth.  But there is no obvious reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all the parts in proper proportion, as soon as any structure became visible in the embryo.  And in some whole groups of animals and in certain members of other groups, the embryo does not at any period differ widely from the adult:  thus Owen has remarked in regard to cuttle-fish, “there is no metamorphosis; the cephalopodic character is manifested long before the parts of the embryo are completed;” and again in spiders, “there is nothing worthy to be called a metamorphosis.”  The larvae of insects, whether adapted to the most diverse and active habits, or quite inactive, being fed by their parents or placed in the midst of proper nutriment, yet nearly all pass through a similar worm-like stage of development; but in some few cases, as in that of Aphis, if we look to the admirable drawings by Professor Huxley of the development of this insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology,—­namely the very general, but not universal difference in structure between the embryo and the adult;—­of parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;—­of embryos of different species within the same class, generally, but not universally, resembling each other;—­of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active and has to provide for itself;—­of the embryo apparently having sometimes a higher organisation than the mature animal, into which it is developed.  I believe that all these facts can be explained, as follows, on the view of descent with modification.