Origin of Species Variorum

Comparison with 1866

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 larvæ 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 like-wise a close similarity in the embryos of different animals within the same class, that we might be led to look at these facts as in some manner necessarily contingent 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." Land-shells and fresh-water crustaceans are born with their proper forms, whilst the marine members of these two great classes pass through considerable and often great developmental changes. Spiders, again, barely undergo any metamorphosis. With almost all insects, the larvæ, whether adapted to diversified and active habits, or remaining inactive, being placed in the midst of proper nutriment or fed by their parents, 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,

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 larvæ 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 organs of importance, excepting those for reproduction.

We are so much accustomed to see a difference in structure between the embryo and the adult, ... that we are tempted to look at this difference as in some manner necessarily contingent on growth. But there is no .. reason why, for instance, the wings of a bat, or the fins of a porpoise, should not have been sketched out with all their parts in proper proportion, as soon as any structure became visible. .. .. .. ↑ In some whole groups of animals and in certain members of other groups this is the case, and 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." .. .. .. .. .. .. .. .. .. .. .. .. .. Land-shells and fresh-water crustaceans are born having their proper form, whilst the marine members of the same two great classes pass through considerable and often great .. changes during their development. Spiders, again, barely undergo any metamorphosis. The larvæ of most insects pass through a worm-like stage, whether they are active and adapted to diversified habits, or are inactive from being placed in the midst of proper nutriment or from being fed by their parents; but in some few cases, as in that of Aphis, if we look to the admirable drawings of the development of this insect, by Professor Huxley,