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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 no trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which fail. .. .. .. .. .. Thus Fritz Müller has .. made the remarkable discovery that certain shrimp-like crustaceans (allied to Penœus) first appear under the simple nauplius-form, and after passing through two or more zoea-stages, and then through the mysis-stage, finally acquire their mature structure: now in the whole great malacostracan order, to which these crustaceans belong, no other member is as yet known to be first developed under the nauplius-form, though .. many appear as zoeas; nevertheless Müller assigns reasons for his belief, that if there ... had been no suppression of development, all these crustaceans would have appeared as nauplii.
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 indivividual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;— the common, but not invariable, resemblance between the embryos or larvæ of the most distinct species in the same class;— the embryo often retaining whilst within the egg or womb, structures which are of no service to it, either at that or at a later period of life; on the other hand larvæ, which have to provide for their own wants, being perfectly adapted to the surrounding conditions;— and lastly the fact of certain larvæ standing higher in the scale of organisation than the mature animal into which they are developed. I believe that all these facts can be explained,
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 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, 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 an 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, 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 ex- plained,