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



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., 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 

 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 

 phenomena 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 indue- 



