388 



W. F. riSKE. 



Competitive enmity between two individual organisms, or two species, mi»ht be 

 indicated diagrammatically as in Fig. 1. The arrows indicate that enmity is mutual, 

 and in any conflict along such lines the better or the stronger individual or species wins. 

 Parasitic or predatory enmity is indicated as in Fig. 2. It is one-sided. The parasite 

 may be the enemy of the host, but the latter cannot be considered as the enemv of the 

 parasite or predator which depends upon it for subsistence. The relations between 

 plants and insects, and between insects and other animals, to which Dr. Harris calls 



(2) 



(4) 



our attention are those which correspond to Fig. 2, and which may be presented more 

 specifically in Fig. 3. He did not recognise competitive enmity. But we certainly 

 cannot secure a clear conception of the relations between different species and between 

 species and their environment unless the struggle for existence between competitors 

 is recognised. This can be done by combining Figs. 1 and 2 to form Fig. 4, in which 

 the parasite is seen to menace competing hosts. Then, whenever natural enmity 

 of the parasite towards its competing hosts is unequally displayed the parasite is very 

 likely to become the real enemy of the one, and the real friend of the other. 



This principle certainly applies to the inter-relations between all plant and animal 

 species, and therefore to the relations between insects and their plant hosts on the one 

 hand and their parasites and predatory enemies on the other. It serves as an 

 explanation for many otherwise obscure phenomena, such, for example, as the dis- 

 appearance of the American cabbage butterfly from the larger portion of its range 

 foUowmg the introduction of a European competitor. Other and numerous examples 

 of its working out were encountered in connection with the attempts to introduce 

 into America the parasites and other " natural enemies " of gipsy and brown- tail 

 moths. The best examples, however, are to be found in connection with insects in 

 their relation to plants. 



