THE DUAL LICHEN HYPOTHESIS. 
121 
It is widely admitted, and has been for many years, that the Colle- 
maceae are aberrant forms of Lichens, even if they be Lichens at 
all. The genera Collema , Leptogium, Synalissis, Myxopuntia , 
Omphalidiuniy Paulia , Lichina , and Mynangium have been named 
together, as forming a group with the thallus of an Alga and the 
fruit of a Lichen. Some have excluded them from Lichens alto- 
gether, others have accepted them with reserve, as aberrant forms, 
and all this was done before any Schwendener hypothesis was 
dreamt of. Montagne and the Rev. M. J. Berkeley at one time 
were disposed to include all these genera under the name of Colle - 
macece, as distinct from Lichens proper. (See “ Lindl. Veg. King.,” 
p. 49). Cohn afterwards excluded them in a proposed arrange- 
ment. Hence we may conclude, that whether they are included 
with Lichens or not, they are aberrant forms, in no sense typical of 
Lichens, and cannot be held as evidence in any dispute as to the 
character of Lichens as a whole. 
Weighed and found Wanting. — It only remains to sum up the 
evidence and apply it to the purpose for which the foregoing re- 
marks were designed. 
It is perfectly legitimate to construct an hypothesis with the 
view of ascertaining the truth, but it appears to me that such an 
hypothesis should be tested in three ways. It should be considered 
as demonstrated, on the one hand, or be rejected, altogether, on the 
other, if it stands or falls by these three tests : 
I. — We should be prepared to ascertain whether our hypothesis 
is consistent with existing scientific facts. 
II. — Whether the hypothesis is sufficient to account for the 
phenomena in question. 
III. — Whether the phaenomena can be produced in any other 
manner. 
I. — As to the harmony of the hypothesis with existing scientific 
facts. The assumption that two separate and distinct organisms 
are combined in one plant, which, by its own proper system of re- 
pioduction, is capable of continuing its species, each individual of 
its piogeny also exhibiting the same phaenomena of assumed dual 
existence, is inconsistent with known scientific facts, because, 
whilst one supposed plant proceeds from its proper germ, the other 
has none, and is, therefore, spontaneously evolved. 
Thus, if the Lichen, by means of sporidia, is capable of pro- 
ducing plants which are, in all respects, like the parent, then the 
sporidium of the fungus element reproduces also the algal element, 
and the same spore, or ovum, is the origin of two distinct and 
widely diverse organisms, i.e. f a fungus and an Alga, which is 
inconsistent with scientific fact. 
The seed of the maple reproduces the Maple tree only, its 
parasite, the Rhytisma , establishes its parasitism afterwards by 
means of its own proper spores. Thus it is with other hosts and 
other parasites. If one germ is the origin of both the dual 
