Origin of Species Variorum

Comparison with 1869

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 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. 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, we see hardly any trace of the vermiform stage.

In some cases it is only the earlier developmental stages which fail; these apparently having been suppressed. Thus Fritz Müller has recently made the remarkable discovery that certain shrimp-like crustaceans (allied to Penœus) first appear under the simple nauplius-form, and passing through two or more zoea-stages, and through the mysis-stage, finally acquire their mature structure: now in the whole enormous malacostracan class, to which these crustaceans belong, no other member is as yet known to be first developed under the nauplius-form, though very many appear as zoeas; nevertheless Müller assigns reasons for his belief that all these crustaceans would have appeared as nauplii, if there had been no suppression of development;— or that they were primordially developed under this form.

How, then, can we explain these several facts in embryology,— namely, the very general, though not universal, difference in structure between the embryo and the adult;— the various parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at an early period of growth alike;— the general, but not invariable, resemblance between the embryos or larvæ of the most distinct species in the same class;— the embryo retaining whilst within the egg or womb, structures which are of no service to it, either at that period or later in life; whilst embryos at a later period, or larvæ, which have to provide for their own wants, are perfectly adapted to the surrounding conditions;— and lastly the fact of certain larvæ standing higher in the scale of organisation than the mature animals into which they are developed. I believe that all these facts can be explained, as follows. .. .. .. .. .. .. ..

It is commonly assumed, perhaps from monstrosities .. affecting the embryo at a very early period, that slight variations or individual differences necessarily appear at an equally early period. We have little evidence on this head, but what we have certainly points the other way; for it is notorious that breeders of cattle, horses, and various

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 larvæ 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 indivividual 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.

It is commonly assumed, perhaps from monstrosities often affecting the embryos at a very early period, that slight variations necessarily appear at an equally early period. But we have little evidence on this head— indeed the evidence rather points the other way; for it is notorious that breeders of cattle, horses, and various