38 



D and E combined from 975 to 1,308. This latter result might 

 lead to an increase of the nnmber of predators in the area, or 

 to an increase of the more cryptic forms of the food of the pred- 

 ators, owing to a greater amount of a more easily procured food. 

 T do not imagine that this is the only method by which an 

 introduced species supplants an endemic form ; many complex 

 causes may lead to the same result. During a discussion on 

 this subject at a former meeting. Dr. Back stated the case of 

 two species of Aleyrodes in Florida attacking Citrus: Aley- 

 rodes citri and A. citri folia. When citri folia is present in 

 quantities in an orange grove and citri is introduced, the latter 

 soon supplants the former as a pest. In this case, Dr. Back 

 stated, there is no reason to consider that parasites, insect or 

 fungus, play any part, but that the" ascendancy of citri over 

 cityifoJia is due more to a slight difference in fecundity and 

 life history. The elucidation of this, and similar problems, is 

 of great interest, and of valne to bionomics as well as economics. 



Specific Chauactrks axd ]\[oktaltty of I^imature 

 TxnivinrALS. 



One of the chief things that has been impressed upon me 

 during many years of observations of the death factors of in- 

 sects, is that the mortality is highest in the immature stages. 

 Natural Selection has but a limited field in the adult stage. 

 It would be easy to quote figures to show this, especially among 

 the Homoptera. Mr. J. C. Kershaw, after many years' study, 

 came to the same conclusion in regard to the Lepidoptera of 

 South China. This fact must be borne in mind when we con- 

 sider the origin of adult specific characters by ISTatural Selec- 

 tion. 



The vast majority of specific characters are such that we 

 cannot conceive of them being selected on account of their 

 utility and they have no connection with the earlier stages of 

 the insect. We have only to take up a JMonograph of some one 

 large genus and note the characters which distinguish the spe- 

 cies from one another ; it is only by insisting u]ion our ignor- 

 ance that we can maintain our belief in their vital utility. Take 

 for instance our genus of Proterhinus; he would be a bold man 

 who would try to maintain that each specific character was of 

 a life-and-death value to its possessor. Or take the genwsrer- 

 \/ l-iusirUa; here are some fourteen species living on the same 



