MR. HENRY WALTER BATES ON PHASMIDÆ, 323 
variously modified in external structüre in the direction of disguises to guard the species 
against extermination by their enemies. It is thus that from the Phasma type have 
been produced the narrow, green or yellow, apterous Bacilli, with their habit of stretching 
out rigidly their elongated limbs when clinging to a plant, so as to assume the appear- 
ance of a slender stem—the flattened and rugged-lobed, mossy-looking Prisopi and 
Creoxyli, which, when the wings are folded, cannot be distinguished from a piece of 
lichen-covered bark—the flattened Phyllium, which, as every one knows, cannot be dis- 
tinguished from a leaf of the tree on which it erawls—and so forth, the adaptation of 
general form carrying with it modifications of limbs and appendages. The difficulty 
which we find in defining generic groups in the family is explicable on the ground that, 
where there is much of this adaptive modification, all the corporeal parts concerned must 
have become to a high degree variable. Natural selection having, from the first, favoured 
the species which offered variations in these parts, the tendency to variability has become 
perpetuated by inheritance. There can be no other explanation of the persistence of cer- 
tain struetures throughout a range of species—the persistence which enables naturalists 
to define generic and other groups—than that, in the long process of multiplication of 
forms in Nature, these parts have not been involved in the process of adaptation. It is 
a strong proof of this persistence having prevailed in Nature, that all naturalists of the 
old school have held so firm a faith in the original creation of genera as wellas of species. 
To state it in simple language, they believed that when a species was created it was en- 
dowed with certain features in common with other allied species: a newly-originated 
form was thus thought to receive its generic as well as its specific stamp. A group like 
the Phasmidæ, therefore, ought, instead of perplexing us, to be regarded with high 
interest, as showing that the existence of generic characters is not a universal rule, and 
as affording a probable explanation why it is not. 
In the following descriptions I have followed the classification of Professor Westwood, 
who adopted its main features from Lichtenstein, Gray, Burmeister, and their successor, 
De Haan, the author of one of the best treatises on the family, comprised in his work on 
the Entomology of the Dutch possessions. The groundwork of this ciassification is the 
gradation in development of the wings from genus to genus. Thus it begins with those 
genera which are wingless in both sexes, these forming one Division, and passes through 
those in whieh the males are winged and the females wingless, or in which the 
wings are rudimentary, to the genera which have well-formed wings in both sexes—the 
whole of the latter forming the second Division. The wingless series commences with 
those forms which have much-abbreviated antennæ and very attenuated bodies, and pro- 
gresses to those having long setiform antennæ, or bodies of much more compact structure. 
The winged series progresses gradually from those genera in which the upper and lower 
wings are either rudimentary or developed in one sex only, to those in which they exist in | 
both sexes (but the upper wings of extreme shortness), ending at length with genera in 
which both upper and lower wings have become elongated in an approach to due propor- 
tion. This classification has lately been severely eritieised by Dr. Gerstaecker, the author 
of the Entomological portion of the Berlin annual Report on the progress of Zoology. 
222 
