PARASITISM v. SYMBIOSIS 137 



a method of life which is sociologically inferior, in the end leads 

 to many deleterious reactions, to physiological weakness, to 

 susceptibility and to disease. 



It is highly suggestive in this connection that, as Prof. Farmer 

 says, the predisposition to infection in plants is probably connected 

 with a disturbance of the photosynthetic processes, i.e., socio- 

 logically speaking, an interference with an essential and widely 

 useful industry, and further, that the " whole matter of immunity 

 is evidently very closely related with nutrition," which again 

 cannot surprise us, as we have now fully seen that a disturbance 

 of nutrition may throw out of gear what is all -import ant, namely, 

 the highly specialised system of mutual industries upon which 

 is based " organic civilisation." 



As Prof. Farmer himself says in the case of flowering parasites, 

 which have lost their chlorophyll : " There is the strongest 

 possible evidence that the change has come about in correlation 

 with the altered conditions of nutrition." 



The chief correlation in this connection, in my opinion, is 

 the sociological correlation, which means loss of biological support 

 and of biological sanction. Nor is it that Prof. Farmer is 

 entirely blind to sociological correlations. As a broad-minded 

 observer, he at least calls attention to the existence of sociological 

 sequence on one or two occasions. The tacit implication, how- 

 ever, is that such occasional sociological illuminations are to be 

 regarded as ornamental rather than real an undue limitation, it 

 seems to me, of the application of Science. 



The following passage bears out my remarks, whilst it may be 

 said at the same time to afford an excellent illustration of the 

 strength and persistence among plants of what I have termed 

 the " symbiotic sense " : 



It is not a little curious that in a large family of plants like the Loran- 

 thaceae, to which both Loranthus and the Mistletoe belong, some species 

 should not have advanced still farther in the parasitic direction. But 

 although nearly all of them draw their water supplies from another plant, 

 they have never taken the final step of absorbing from it the organic food. 

 They have consequently, or perhaps one should say correlatively, retained 

 their leaves, and all the complexity of structure which, as we have seen, 

 the presence of the green leaf entails. 



Evidently the retention of some degree of status by a plant 

 is not compatible with large steps in the parasitic direction, with 

 lazy indulgence in food, or with a surrender of the strenuous 

 symbiotic sense which at the same time makes for forbearance 



