without olfactory nerves, while the rest of the cetacea and even fishes have them: there must be a reason which determines this. Individual real exceptions to this universal law of design in organised nature have indeed been discovered, and with great surprise; but in these cases that _exceptio firmat regulam_ applies, since they can be accounted for upon other grounds. Such, for example, is the fact that the tadpoles of the pipa toad have tails and gills, although, unlike all other tadpoles, they do not swim, but await their metamorphosis on the back of the mother; that the male kangaroo has the marsupial bones which in the female carry the pouch; that male mammals have breasts; that the _Mus typhlus_, a rat, has eyes, although very small ones, without any opening for them in the outer skin, which thus covers them, clothed with hair; and that the moles of the Apennines, and also two fishes—_Murena cœcilia_ and _Gastrobrauchus cœcus_—are in the same case; of like kind is the _Proteus anguinus_. These rare and surprising exceptions to the rule of nature, which is otherwise so rigid, these contradictions with itself into which it falls, we must explain from the inner connection which the different kinds of phenomena have with each other, by virtue of the unity of that which manifests itself in them, and in consequence of which nature must hint at some thing in one, simply because another of the same type actually has it. Accordingly the male animal has a rudimentary form of an organ which is actually present in the female. As now here the difference of the _sex_ cannot abolish the type of the _species_, so also the type of a whole order—for example, of the batrachia—asserts itself even where in one particular species (pipa) one of its determinations is superfluous. Still less can nature allow a determination (eyes) which belongs to the type of a whole division (Vertebrata) to vanish entirely without a trace, even if it is wanting in some particular species (_Mus typhlus_) as superfluous; but here also it must at least indicate in a rudimentary manner what it carries out in all the others.