PARASITISM v. SYMBIOSIS 135 



need by no means to be read as in any way derogatory to the 

 high function of Symbiosis. On the contrary. The writer has 

 merely demonstrated in other words that the requirements of 

 Symbiosis in the vegetable world are : work, photosynthesis, 

 reciprocity and commensurate " cross-feeding." Now Para- 

 sitism antagonises every one of these factors. How can it be said 

 to approximate to Symbiosis ? Evidently the most essential 

 industry of all, that of photosynthesis, imperatively demands 

 cross-feeding, as a guarantee of continuity. Shall we then be so 

 ill-advised as almost to identify Symbiosis with Parasitism because 

 at the lower rungs of evolution, or in associations comprising the 

 most primitive of organisms with little development of 

 " character " the risk is that the delicate requirements of 

 Symbiosis may be occasionally infringed ; because abuse of power 

 is a common occurrence, and because transitions from a life of 

 strenuous labour to one of unholy idleness are always possible ? 

 Though it be quite true that any infringement of its rules 

 interferes with Symbiosis and makes the association pro tanto 

 " unpayable," yet we have seen that permanence of healthy 

 association is in the path of Symbiosis and of Symbiosis alone. 

 There is, of course, more or less " payability " in the different 

 forms of Symbiosis, and the least " payable " forms may be said 

 to approximate the least offensive forms of Parasitism. But, 

 as I have said before, we should not confine our attention to those 

 cases of Symbiosis which hover on the borderland of Parasitism. 

 We should study Symbiosis in its unattached and collective forms 

 in order to obtain a just and comprehensive estimate of its 

 significance, its value and of the way it is sanctioned by Nature. 

 What Prof. Farmer says on another page respecting the immense 

 importance of the elaboration of chlorophyll, that it is " fraught 

 with consequences to the whole organic world compared with' 

 which all the other structural products of evolutionary change 

 sink into insignificance and obscurity," applies with equal force 

 to the importance of organic reciprocity and of the necessarily 

 implied cross-feeding. 



Work, reciprocity, and cross-feeding, these are the factors 

 constituting " la vie normale," notwithstanding all interferences 

 to the contrary. We may glean further confirmation of this 

 truth from the following of Prof. Farmer's statements : 



Lichens (he says) are particularly instructive in showing that the 

 form assumed by an organism is in the long run determined by the chemical 



