122 
THE DUAL LICHEN HYPOTHESIS. 
elements in a lichen, then these elements must both be considered 
as parts of a single individual. 
The theory assumes, further, that a fungus is parasitic upon a 
smaller and weaker organism, which it does not injure or destroy. 
This, again, is contrary to scientific fact, as it confers upon, or 
assumes for, a destructive force, conservative powers. 
The hypothesis assumes also that all the substance of all lichens, 
except the gonidia, are fungi, whereas, as has been shown, their 
structure, composition, habits, &c., &c., all differ materially from 
fungi, to such a degree, that they cannot be regarded as identical. 
The hypothesis assumes also that the gonidia are certain forms 
of other plants, which they externally somewhat resemble, whereas 
the gonidia are organs of the lichen, performing definite functions 
in its behalf. 
Hence, briefly, there remains no other conclusion but that the 
hypothesis fails to support the first test, because it is not consis- 
tent with existing scientific facts. 
II. — Whether the hypothesis is sufficient to account for the 
phaenomena in question. That is, whether the combination of a 
fungus and alga is sufficient to account for the production of a 
lichen, or to state the hypothesis in the phraseology of its advo- 
cates, whether a fungus parasitic on an alga is a sufficient cause to 
account for a Lichen. 
Unless it can be shown that the fact of its parasitism is sufficient 
to alter the entire character of a fungus, it is not a sufficient cause 
to account for the existence of Lichens. Unless it can be admitted 
that parasitism will convert a fugitive, short-lived fungus into a 
perennial lichen, induce it to live on a dry, barren rock, which, as a 
fungus, it could never accomplish, to support great extremes of 
heat and cold, to submit to frost and snow without destruction, to 
flourish on growing trees, whilst still vigorous, and without induc- 
ing decay, to assume a fibrous structure instead of a cellular, to 
produce certain acids within its tissues, which are unknown to it 
in other conditions, to exchange a putrefactive tendency for a 
persistent one, to abandon for ever a career of pertinacious des- 
tructiveness for one of inoffensiveness, in short, to reverse its entire 
character, unless it can do all these things as a result of parasitism 
on the minute green cells of an Alga, then the fact of such a para- 
sitism is insufficient for the production of the phaenomena. 
Inasmuch as we have no experience of such a total change in the 
essential characters of fungi under any conditions of climate, or 
local circumstances, we cannot admit that simple parasitism will 
cause such a change. Numerous instances of parasitism are found 
habitually to occur amongst fungi. Many species exist in no other 
form than as parasites, and yet no alteration in the direction of 
lichens is known to take place. Therefore we are compelled to 
adopt the conclusion that the hypothesis is insufficient to account 
for the phaenomena in question. 
XII. — Whether the phaenomena can be produced in any other 
