M. C. COOKE ON THE DUAL-LICHEN HYPOTHESIS. 181 



experiences of the Collemaceae, are essentially unsatisfactory. It is 

 widely admitted, and has been for many years, that the Collemaceaa 

 are aberrant forms of Lichens, even if they be Lichens at all. The 

 genera Collema, Leptogium, Synalissis, Myxopuntia, Oniphalidium, 

 Paulia, Lichina, and Myriangium 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 altogether, 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 Collemacece, as distinct from 

 Lichens proper. (See " Lindl. Veg. King.," p. 49.) Cohn 

 afterwards excluded them in a proposed arrangement. 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 phenomena 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- 

 production, is capable of continuing its species, each individual of 

 its progeny also exhibiting the same phenomena 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- 



