An History of Fungusses, growing about Halifax (Vols 1-2)

by examining all the parts in a combined or complicated scene. And when a plant is discovered, concerning which we are doubtful whether it is a species of itself or only a variety of a species already known, we ought to carefully compare it with the original from which we suppose it may possibly be derived. If we find it to agree in the number, figure, and situation of its parts, and that the matter of which it is composed agrees in substance or texture, we may reasonably suppose that the doubtful plant is a variety and not a new species, even though it may be much different in respect to time, soil, magnitude, colour, etc.

On the contrary, though two plants may agree in colour, time, soil, situation, size, smell, taste, etc., yet if they disagree in any one essential mark, we are nevertheless certain of their specific distinction. For instance, suppose the Agaricus fimetarius has an upright white stem growing from a bulbous root which is destitute of a volva; the Agaricus annulatus also has this. Suppose it further to have a large white pileus, brown at the apex, and covered here and there with soft brown scales; suppose it to have a white curtain and gills of a pale flesh colour while young, changing their colour afterwards; the Agaricus annulatus exhibits all the above characters as evidently as the other. Thus far, therefore, no certain specific mark is discoverable; but on examining them a little further we find that the gills of the fimetarius are arranged in one series only, being all of equal length and extending from the rim of the pileus to the top of the stem. But in the annulatus they are arranged in several series, unequal in length, which is a certain mark of distinction between the two species. This one diagnostic Diagnostic: a distinguishing characteristic or symptom used for identification. being found, we then support it with evidence drawn from habit, figure, soil, size, duration, smell, taste, colour, and dissolution; all of which, being properly attended to, will—with the assistance of application to the study and actual observation made upon the plants in the different stages of growth—render the knowledge of their specific distinctions both easy and certain.

To explain a little further what I mean by "aids to support the specific mark" in the above instance, the pileus in the Agaricus fimetarius is of