Limits of Species in the Diatomacese. By Thomas Cornier. 459 
criterion of “ species ” is probably due to its hardly ever being directly 
applicable. However efficient in theory, it is in practice very seldom 
capable of experimental proof. Only in the case cf domesticated or 
cultivated species, or in those few genera, such as Salix and Ver- 
lascum, in which intercrossing habitually takes place in a state of 
nature, is it directly available as a test. Indirectly, however, it is 
more widely applicable. As perfect fertility must result in the pro- 
duction of an unbroken series of intermediate forms, conversely the 
contemporaneous existence of such a series may fairly be held to 
indicate the power of producing fully fertile progeny. Consequently, 
even the extremes of such a series should be regarded as constituting 
nothing more than varieties of a single species. This test, though 
indirect, is available for purposes of classification ; and it appears to 
me that, while the direct criterion enables us to form a definite con- 
ception of what a natural species is, and what a systematic species 
ought to be, the indirect application suffices to establish practical 
limits for the latter. 
It will be seen that what Kerner terms “ good ” and “ bad ” species 
may alike be due to a “hiatus” between one series of forms aDd 
another ; but while in the latter case the hiatus is merely the result 
of imperfect observation on the part of naturalists, in the former it 
arises from cross-sterility established by Nature itself. 
No justification is afforded for any gradation of species, such as 
was proposed by Mr. H. C. Watson,* into super-species, ver-species, 
and sub-species. The last are described by Mr. B. Syme | as differ- 
ing “ from ver-species only in having the distinctions between them- 
selves slighter, or less generally recognised, or in apparently shading 
off more gradually into one another.” If this “shading off*” consists 
of an unbroken series of intermediate forms, the “ sub-species ” may 
be more than usually pronounced varieties, but are still only varieties. 
If there is a gap in the series, the forms separated by it should be 
ranked as species, at any rate until such time as additional forms, 
filling up the gap, may be subsequently found to exist. 
No better example of the working of the criterion can be given 
than Bubus fruticosus , so familiar to British botanists. Of this 
group Mr. Syme remarks that, “although the extreme forms are 
widely different, they are so completely connected by intermediate 
ones that I find it utterly impossible to separate them into any 
groups answering to the usual idea of a species.” i Yet he admits no 
less than 41 sub-species. Of the same group Sir J. D. Hooker ob- 
serves, “ Whereas in the fruticose Bubi the four or five most dis- 
tinct British forms are connected by so many links that various 
excellent botanists regard them as forms of one species ; in Bosa , on 
the contrary, the five most distinct British forms are connected by so 
few (comparatively) intermediates, that no good botanists have 
* ‘ Cybele BritaDmca.’ t Engl. Bot., 1863. 
J Op. cit., iii. (1864) p. 162, footnote. 
