The great number and variety of the objects to which the term Adventitious Product, de ffned in the manner we have just proposed, will apply (from a microscopical crystal, for instance, to the highest species of intrinsicallf vegetative Growths) render it necessary, in limine, to introduce some order into the sub ject. We shall consequently set out by tracing those lines of distinction which separate from each other the various objects united together by the cominon property of Adventitiousness.
It would, no doubt, be desirable and most strictly logical to employ some one uniform principle in establishing the various divisions and subdivisions of this, as of all other groups of natural objects, which require classification. But in the present state of know ledge, at least, systematic accuracy of this kind isunattainable. Neither the anatomy of texture or of form, the physical or chemical nature or properties of ultimate elements, the mode of formation, the physiological properties, nor the patholog,ical influences of morbid products, will, taken singly, supply a feasible instrument of classifica tion. All must by turns be made to contribute their share in the work. And as all previous modes of arrangement have been found to bear the impress of contemporary physiolo gical doctrines, so will the existing impulse towards micrological study be traced in ours.
But we have not pushed the use of micro scopical characters to extremes, persuaded as we are that more has been done to lower than to raise micrology in general estimation by the attempt to Make it (in its present.unforined
state) the essential and sole groundwork of distinction of organized products.
Adventitious products present themselves in the solid, the liquid, and the gaseous states ; and this difference of molecular condition co incides with so many pathological distinctions, that (although some objections may on " tran scendental " grounds be raised to the pro cedure,) we shall found upon it a division of the whole into three corresponding groups. A complete description of the Morbid Anatomy of the more complex of the species composing these groups should, we conceive,* comprise that of their material or physico-chemical characters ; of their origin, progress, and de cay ; of their intrinsic morbid changes, (for their lives, as the lives of the organism they inhabit, are liable to variations of health and disease,— they are microcosms within a ma crocosm ;) of the textural alterations they produce in contiguous parts ; and of the modifications their existence entails on the solids and fluids of the economy at large. It is clear, however, that a plan so extensive as this could not be ventured on in the present work ; but, as far as is reasonable, we shall pursue it.