40 
Greville, on the genus Auliscus. 
idea of a species to an extent unknown among the higher 
orders of vegetables. For example^ let a represent a species 
of diatom. By some unknown cause one of its progeny, 
becomes so changed as to constitute a well-marked variety. 
Another of its progeny, c, undergoes a different but equally 
decided change ; and possibly the same thing may occur in 
others. Now these varieties or aberrations from the typical 
condition may be propagated, according to the late Professor 
Smithes calculation, at the rate of a thousand millions in a 
single month. Then, as there is no reason why b and c 
should not also have an indefinite number of nonconformist 
children, all removed in one character or another a second 
stage from the type, and producing duplicates by thousands 
of millions, it is manifestly impossible to say where the con- 
fusion is to end. But this is not all. By the process of con- 
jugation, what Mr. Thwaites calls '^sporangial frustules,^ 
are produced, which are very much larger than the ordinary 
size of the parents; and these, it is presumed, multiply equally 
freely by self-division, and are equally liable, from accidental 
causes, to have their deviation from the normal type perpe- 
tuated. Such is the theory ; and to arrive at anything like 
fixed specific distinctions would seem to be almost a hopeless 
endeavour. Nevertheless, by correcting processes unknown 
to us, we cannot doubt that the typical characters of real 
species are preserved.^^^ 
There is, besides, another element to be taken into account 
connected with the process of conjugation above referred 
to. Professor Smith remarks, " Cases have fallen under my 
notice which seem to indicate that the further process of re- 
production consists in the resolution of the contents of the 
sporangium into a ' brood ^ of diatoms having the same form 
and specific characters as the original frustules which origi- 
nated the sporangia.^^t And he adds, in speaking of Cocco- 
nema Cistula, " forms of every size intermediate between the 
minutest frustule in the cyst and the ordinary frustules en- 
gaged in the conjugating process were easily to be detected ; 
and the conclusion was inevitable, that the cysts and their 
contents were sporangia of the species with which they were 
associated, and indicated the several stages of the reproductive 
process.^-' Every diatomist must be familiar with similar 
broods of Cocconeides, looking just like broods of young 
spiders. Now, the chief point of interest here is, what be- 
comes of these broods? How do they increase in size? 
Minute as the individuals are compared with the parents^ 
'E(tiii New Phil. Join'./ vol. new series, p. 26. 
t 'Biit. Diat.,' vol, ii, p. 15. 
