M. C. COOKE ON THE DUAL-LICHEN HYPOTHESIS. 179 



for its existence, then Nature has made a mistake in making it 

 dependent on another being for its existence. 



Here, then, we have clear demonstration that the gonidia are 

 developed within the substance of the Lichen itself in a determinate 

 and uniform manner ; that, instead of being altogether foreign from 

 the Lichen, they are generated within it, and hence (according to the 

 hypothesis J the parasite produces from its own substance the host 

 upon which it is parasitic. 



The admirable Memoir of Tulasne should be consulted in conjunc- 

 tion with Dr. Minks' contributions to the " Flora," and it will be 

 seen that the latter confirms and amplifies the former, adding thereto 

 new facts, and thus, doubtless, establishes the true relationship of 

 gonidia to the residue of the Lichen. 



Is Parasitism proven ? — A parasite is usually defined as an animal 

 or plant which lives upon or attaches itself to another, and derives 

 therefrom sustenance and support. The misletoe, which attaches 

 itself to poplar and other trees, is a parasite. The mould which attacks 

 and destroys potatoes, is a parasite, entozoa are parasites, and so 

 also are the pediculi, and other insects which infest the higher 

 animals. In ordinary conversation there is seldom likely to be any 

 misconception of what is a parasite. In all known instances of 

 parasitism, it is, I presume, admitted that the parasite thrives and 

 flourishes at the expense of its host. In every known instance of 

 parasitic fungi the fungus lives at the expense of its host, which it 

 injures and ultimately destroys. Schwendener himself says, " the 

 vine and potato fungus, as well as all other fungi which vegetate in 

 living organisms, destroy their host plant, or host animal, in the un- 

 equal struggle." This universality of destructive parasitism in fungi 

 knows no exception, and is, therefore, equivalent to a natural law. 

 Any hypothesis which depends upon fungal parasitism, must of 

 necessity recognize this fact, and conform to it. The fact cannot be 

 reversed to save the hypothesis. 



Our experience of fire, in whatever form it occurs, is that it 

 burns or consumes that upon which it operates. We have no ex- 

 perience of fire which does not support itself by causing that 

 change of form and condition which we term combustion. Hence 

 we are justified by experience in rejecting any theory based upon a 

 reversion of this fact, that is, on the assumption that fire does not 

 consume. 



Inasmuch as the supposed fungus, said to be parasitic upon the 



