SPECIES. 
57 
acter of the hinge, etc., are all important features in the discrimina- 
tion of species; the anatomical criteria differ in different groups, but 
ill the Gastropoda reliable features for the differentiation of species 
are often found in the modifications of the different parts of the 
reproductive system, etc. 
Some species are remarkably constant and more persistent in the 
retention of their characters than others; these inflexibly organized 
forms do not readily accommodate themselves to, or vary in 
response with the special nature of the environment, and are usually 
restricted in their distiibutiou and difficult to acclimatize; according 
to Dr. 0. Boettger, species of this character are ancient forms, which 
have existed from tertiary times, and are found fossilized in the 
deposits of that period, possibly these species have passed through 
their period of variability or specific youth, and have entered upon 
the period of decadence preceding their final e.xtinction. 
Others species are remarkably variable, and readily and quickly 
modify and adapt themselves to any change in their locality or sur- 
roundings; species of this character are according to Dr. Boettger, 
of comparatively modern origin, and are not found fossilized in the 
deposits of tertiary age and have therefore been presumably evolved 
from some pre-existing form since that period, the exuberant vari- 
ations in form, size, colouring or texture they exhibit, are possibly 
evidences of their recent evolution and the youth of their specific life. 
The opinion has within recent years gained ground amongst 
scientists in this country, that unless closely allied forms present 
some internal organic difference, they cannot rank as species. This, 
in my opinion, though an undoubtedly tiue and scientific test, 
should yet be applied with judgment and discrimination and not 
indiscriminately applied to every case, as the shell is just as truly 
an oi’gan of the animal as the penis, spermatheca, dart, or 
buccal armature, and this being so, the shell becomes a valid criterion 
of difference, the only criterion if other sufficiently distinctive organic 
modifications are absent. If this is not so, then to be consistent we 
must unite under one specific name such forms as Helix hispida and 
Helix rufescens, as the chief differences between these species are 
testaceological rather than anatomical. Even Simroth with his vast 
experience cannot really decide, whether in certain genera of slugs, 
the external or internal characteristics possess the greater specific 
value and constancy. 
