58 I'March, 



students may render our knowledge of the occurrence of even a whole 

 Order very incomplete. To alter a little a statement wliich used to be 

 applied to the Goleopfera : " Our knowledge of the distribution of the 

 Diptera (say) is really our knowledge of the distribution of the 

 Dipterists." Even where students are reasonalily numerous, an insect 

 may escape observation because of its close resemblance to some com- 

 moner form, and this more especially when the species is small. This 

 applies, for example, among beetles, probably to certain of the smaller 

 O.ryteli, the Athefae, and the Trichopterygidae. Finally, our ignorance 

 of the bionomics of a species may frequently produce a totally in- 

 correct notion as to its commonness. We have had interesting illlus- 

 trations of this in the case of the insects connected with the nests of 

 moles and birds,* and of those connnected with burnt timber,t and it is 

 just possible that certain species, such as Lebia crux-minor and Rhizo- 

 phagus aeneus, which have been taken in widely separated localities, 

 may turn out to be not really uncommon when we know their habits 

 and life-history. For example, it has been suggested that the latter 

 insect occurs under the bark of alders, which are frequently sub- 

 merged, though I am not aware that anybody has systematically 

 tested this statement. Moreover, as my friend Mr. W. E. Sharp 

 points out {in Hit.), the adult stage of an organism may have a very 

 brief existence, e.g., in the Ephemeridae, so that unless an observer 

 happens to be " on the spot " at the exact time of its appearance 

 a really common or even abundant species may be rarely seen. 



Grenerally speaking, the factors determining the frequency of a 

 species may be divided into two classes — those connected with its 

 phylogeny, and those connected with its ontogeny ; in other words, 

 those which have determined the existence of the species as a whole, 

 and those which determined the existence of each individual in that 

 species. 



To consider the former first, the rarity or localisation of a species 

 itiay be due to one at least of a number of causes which are fairly 

 easy to distinguish from one another, if observations are made over a 

 suflicienty wide area, and during a sufficiently long period of time. 



1. — Distance from Centre op Origin. 



It is one of the tenets of zoogeography that each animal and 

 vegetable species originated in (as at least, an almost invariable rule) 



* Norman H. Joy, •' Culioptmi in the Ne.sts of Mamuaals and Birds," Ent. Mo. Mag., 190(5, 

 pp. 198, 202, 237-243. 



t G. C. Champion, " A. Bupnstid and other Coleoptfra on I'iiies injured bv heath fires, in 

 N.-W. Surrey," I.e., 190;», jjp. 247-250. 



