46 Journal of the Mitchell Society [Decoy^her 



the same situations. So much we know but there remains a vast deal 

 more to be found out. Are these species limited in their zones by the 

 amount of submergence that they can withstand? What are the 

 physiological characters that make it possible for these species to sur- 

 vive such an environment whereas their generic relatives are confined 

 to xerophytic hal)itats? These and many other questions we might 

 ask but only time and an unlimited amount of field work can give the 

 answer. 



There is another ecological relation that it seems to me would be 

 worth pausing to consider and that is the relation of insects to their 

 hosts. The relation of insects to their animal hosts I have discussed 

 in another connection under parasitism so that there is left for our con- 

 sideration at this point the relation between insects and their plant 

 hosts. This is a broad subject in itself and one deserving of more con- 

 sideration than I am able to give it here. 



Some insects are so limited in the selection of their hosts that it is 

 safe to identify the insect from the host on which you find it; while 

 other insects apparently will eat anything that is green. We have in 

 some cases whole families of insects limited to a narrow group of plants 

 in their host relations, for example the Coleopterous family Bruchidae 

 confines its attacks to the seeds of the Leguminosae. On the other 

 hand we have species that are structurally closely related feeding on 

 widely diverse plants. In fact these relations are so marked that they 

 have been used to correct our phylogenetic notions. Thus the botanist 

 may use insect species in determining the relationships of his plant 

 forms and the entomologist frequently corrects his taxonomic concepts 

 on a purely botanical basis. Naturally such relations cannot fail to 

 impress the serious student and leave him with a vague feeling of dis- 

 satisfaction at his failure to answer them. Thus I have puzzled mj^ 

 mind for many hours in the hope that I might be able to answer the 

 question, why does the gloomy scale occur on soft maples in destruc- 

 tive numbers yet be practically absent from hard maples? Is it 

 simply a difference in the thickness of the bark in the two groups of 

 maples, or is it a difference in the chemical composition of the sap? 

 Whatever the answer, the selection is remarkably clear cut because 

 we frequently see rows of hard maples with a few soft maples inter- 

 spersed in which the hard maples are free from scale and thrifty where- 

 as the soft maples are dead or dying. 



There is another series of ecological relations that is deserving of 

 more than passing attention. I have already referred to that com- 



