M. C. COOKE ON THE DUAL-LICHEN HYPOTHESIS. 183 



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 phenomena. 



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 

 lor the phenomena in question. 



III. — Whether the phenomena can be produced in any other 

 manner. It is almost needless to say much under this head, having 

 already shown, as we proceeded, that all the phenomena of lichen 

 life are wholly compatible with the assumption that these organisms 

 are, in themselves, true, simple, autonomous plants. If, also, we 

 are right in the conclusion that the hypothesis fails when submitted 

 to the two preceding tests, the natural inference will be from the 

 arguments already adduced, that all the phenomena which any 

 Lichen exhibits are compatible with a simple organism. The 

 Theorists admit, by their hypothesis, that all the phenomena, 

 except the gonidia, are wholly compatible with a plant which they 

 call a Fungus. Although we do not admit that Fungus is the 

 correct name to be given to the plant, we also affirm that it affords 

 evidence of being a simple plant, but with the addition of the 

 gonidia, which we hold to be as possible as conidia enclosed within 

 the substance of a Fistulina or a Polyporus, and not only possibly, 

 or even, probably, organs of the Lichen, but that positively such is 

 the case. We require no counter hypothesis, therefore, to support 

 our view, but urge, on the contrary, that where an organism is seen 

 to exist, and continues to reproduce itself in conformity with a 

 certain type, it is folly to attempt, by multiplying causes, to account 

 for phenomena in an abnormal manner, which can be readily 

 accounted for by normal causes. 



