L. Harrison 
9 
diagnostic details, such as the structure of the anterior part of the head, 
and of the genitaHa. Measurements need only include total length 
and breadth, and the length and breadth of the main divisions of the 
body. I attach no value to the measurement of every article of an 
antenna, or of a tarsus, to three decimals of a millimetre, because the 
possible margin of error is so large as to absolutely vitiate the results. 
Finally, I should like to add a few words on the future classification 
of Mallophaga. We have to deal with a group that, owing to its 
biological conditions, has suffered what I might call a retarded evolution. 
There is a general uniformity concealing an infinite number of small 
variations. Because these variations have been small, they have 
never been considered of generic value. But I claim that, if we are 
ever to put the systematics of the group upon a satisfactory basis, we 
must accord it special treatment, on account of its special biological 
relations. We must allow more value to small differences than 
is allowed among groups of free-living insects. The present large 
genera of Mallophaga are absurd accumulations of heterogeneous 
forms. A species is a Lipeurus, or a Degeeriella, simply because it is 
just so long and just so broad, and has, or has not, a crotchet upon 
some segment of the male antenna ! My own firm conviction is 
that the genus Lipeurus alone includes almost as many groups of real 
generic value as there are generic names in my Section iii. 
It seems to me that the most valuable kind of systematic work in 
the immediate future will be close revisional work upon limited groups 
of species. I have been working myself upon the Lipeuri of petrels, 
and find that, with a general facies that is easily recognised, there are 
at least eight groups of generic value, each containing a number of 
species. I find, moreover, that several perfectly valid species are at 
present confused under a single name, in several instances. Waterston 
finds the same condition among the species of Degeeriella from plovers. 
Groups such as the Degeeriella fusca group from hawks, or the 
Philopterus icterodes group from ducks, are not single species, but are 
capable of being separated, by careful work, into a number of species. 
Such work is of more importance at the present time than the 
description of casual new species, unless the latter present particularly 
striking and important features. 
I have to offer my thanks to several friends and correspondents for 
help of various kinds, among whom I may mention especially Messrs 
James Waterston, Imperial Bureau of Entomology; Bruce Cummings, 
British Museum ; and G. F. Ferris, Stanford University. To Professor 
Nuttall, who has given me a free hand to indulge in such typographical 
