Origin of Species Variorum

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,