to the most simple for digesting vegetable food, so those of the bul-
lock, camel, and whale, are links from the ruminating to the most
simple stomachs for digesting animal food; and the camel's stomach
is the most important link in each series, the contraction peculiar to
its fourth cavity making it intermediate between the bullock and the
whale.
Although the above facts appear to throw some light on the digestion of difi'erent kinds of food, they also present difficulties which must remain to be explained when further progress has been made in the investigation. It is in general admitted, that animal substances do not require so long a process to convert them into cher as vegetables; and hence the stomachs of carnivorous animals are in general most simple : but why the whale tribe, which live on fish that are very readily converted into chyle, should have a more complex stomach, it is not easy to explain. What further uses, in regard to other secretions, these preparatory stomachs may have, are foreign to the design of the present paper, which Mr. Home considers as a continuation of a series of observations on digestion, and hopes to extend further at some future opportunity.
An extraordinary diversity of opinion having prevailed amongst naturalists most capable of correct observation, respecting the production and subsequent state of the bark of trees, Mr. Knight has undertaken to investigate the subject : but such are the difficulties of the subject, that, in a course of experiment which has occupied more than twenty years, he has scarcely felt himself prepared, till the present time, even to give an opinion of the manner in which the cortical substance is either generated in the ordinary course of its growth, or re-produced when that which previously existed has been taken off.
Du Hamel had shown, that the bark of some species of trees is readily re-produced when the decorticated albumum is secluded from the air. Mr. Knight has repeated these experiments on the apple, the sycamore, and some other trees, with the. same result; and has also observed, that the wych-elm, in moist and shady situations, will frequently re-produce its bark when no covering whatever has been applied.
A glairy fluid (as Du Hamel justly observes) exudes from the surface of the alburnum, which appears to change into a pulpous organized mass, and subsequently becomes organized and cellular,—facts which are extremely favourable to the opinion of Hales, that the bark is derived from the substanhe of the albumum. But other facts may be adduced which lead to a contrary conclusion; since the internal surface of pieces of bark, when detached from contact with the albumum, but remaining united to the tree at their upper
