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Transactions of the Society. 
reduced them to one species.’ 7 * In treating of the New Zealand 
forms of Veronica, Sir J. D. Hooker writes, “ The species are ex- 
ceedingly difficult of discrimination, present numerous intermediate 
forms between many most distinct-looking ones, and hybridise most 
freely. . . . Between the first 19 species it is most difficult to draw 
any contrasting specific characters ; they appear to present a graduated 
scale of forms.” t The two last extracts are quoted because the 
recognition of only one species in the case of Bubus, of 19 in that of 
Veronica , serves to illustrate the varying sense in which even the 
same author uses the term species. 
Turning now from the general question of species throughout the 
Animal and Vegetable Kingdoms to the much narrower one of 
species amongst Diatoms, we are met, so far as theory is concerned, 
with one initial difficulty. Nothing is knowrn of any process of sexual 
reproduction amongst these minute organisms. We cannot even be 
confident that the process of conjugation between two individuals, 
which has been observed amongst them, does more than approach a 
really sexual method. | Until this has been ascertained, and the 
course of conjugation has been traced throughout, at any rate in some 
instances, we can only proceed on the assumption that as, among the 
higher plants, a differentiation of the sexual elements prevents cross- 
breeding, and thus produces a break in the series of intermediate 
forms, so, among the diatoms, a corresponding differentiation of the 
elements which coalesce in the process of conjugation brings about 
similar results. On this assumption we may adopt, as the practical 
criterion of specific distinction, the existence, or non-existence, of an 
unbroken series of intermediates. This appears to me to be, at any 
rate, a more reliable criterion, and to rest upon a more scientific 
basis, than the purely arbitrary system which has of late been adopted, 
of regarding every difference, however slight, as sufficient grounds for 
constituting a new species. It certainly presents a consistent means 
of classification. 
There is no order of plants in which the “ shading off ” of one 
reputed species into another is more frequent, and in which, conse- 
quently, there has existed a wider divergence of opinion as to what 
constitutes a species. As an instance, it is only necessary to note 
that Mr. Battray quotes, as synonyms of Actinocyclus Ehrenbergii , 
no less than 119 of Ehrenberg’s species; and yet, while he himself 
enumerates 49 species of Actinocyclus , and 290 species of Coscino- 
discus , another diatomist, Mr. Cox, reduces all the species of the two 
genera, 339 in all, to only seven species ; and Prof. Van Heurck, 
in his recent treatise on the Diatomacege,§ quotes this opinion with 
approval, so far, at least, as the genus Coscinodiscus is concerned. 
* ‘Student’s Flora,’ ed. 1870, p. 120. 
t ‘ New Zealand Flora,’ 1864, p. 2-04. 
X The late Mr. Buffham is the only observer who has described, as occurring in 
a diatom, a conjugating process which he regards as “ distinctly sexual,” and as 
forming a step towards . . . the sexual process in those “ algae which produce 
antherozoids.” , § P. 101. 
