higher mammalia; but in no instance is there an exact identity between any of these. It is to be remembered, that every animal must pass through _some_ change in the progress of its development, from its embryonic to its adult condition; and the correspondence is much closer between the embryonic fish and the foetal bird, or mammal, than between these and the adult fish."--(P. 196.) And take, also, the following short passage from the preface of the same work, where the author has been speaking of the latest discoveries of physiologists on the development of the embryo. "Thus, when we ascend the scale of being, in either of the two organized kingdoms, we observe the principle of specialisation remarkably illustrated in the development of the germ into the perfect structure. In the lowest of each kind, the first-formed membranous expansion has the same character throughout, and _the whole enters into the fully-developed structure_. In higher grades the whole remains, but the organs evolved from the centre have evidently the most elevated character. _In the highest none but the most central portion is persistent_; the remainder forming organs of a temporary and subservient nature." The fact that the animal kingdom exhibits a gradual progression from forms the most simple to forms the most complex, is, of course, appropriated by our author as a proof of his theory of successive development. It is well known, that whilst this scale of being is an idea which occurs to every observer, the naturalist finds insuperable difficulties in arranging the several species of animals according to such a scale. To relieve himself from these, the author has taken under his patronage what, in honour of its founder, he calls the _Macleay System_, in which the animal kingdom is "arranged along a series of close affinities, _in a circular form_;" into which circles we will excuse ourselves from entering. It is a system as confused as it is fantastic; and our author, who writes in general in a clear and lucid manner, in vain attempts to present us with an intelligible exposition of it. Arrange the animal creation how you will, in a line or in circles, there is one fact open to every observer, that however fine may be the gradations amongst the lower animals, the difference between the higher animals is very distinctly marked. It is a difference which does not at all accord with the hypothesis of our author, "that the simplest and most primitive type gave birth to the type next above it, and this again produced the next higher, and so on to the very highest, the stages of advance being in all cases very small--namely, from one species only to another; so that the phenomenon has always been of simple and modest character." Whilst he confines himself to _mollusks_, and suchlike obscure creatures, the phenomenon he supposes may not be very