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Transactions of the Society. 
abandoned. Coscinodiscus ( Odontodiscus ) excentricus is a good 
example, for it exhibits a complete gradation from the entire absence 
of teeth, to a thick-set coronet. 
The natural result of the adoption, for specific diagnosis, of such 
characters as have been mentioned above, has been the undue multi- 
plication of specific names, and the overburdening of diatom nomen- 
clature with an enormous mass of synonyms. Apart from the too 
frequent description of altogether identical forms by different authors, 
under different names, specific designations have been given to forms 
differing from each other only in characters which are quite inconstant. 
Some observers justify the practice, maintaining that it is convenient 
to have distinctive names for forms, however unimportant the points 
in which they differ from each other, without entering into the 
question as to what constitutes a “ species.” 
Fortunately, that is a question into which it is quite unnecessary 
for us to enter. All that we have to consider, in connection with our 
present subject, is the much simpler one, what does not constitute a 
species. The two valves of an oyster differ considerably from each 
other, but none would suggest that they belong to two species, or 
that it is desirable to give them different specific names. The 
plumage of a young bird is sometimes very unlike that of the adult 
form ; but it is not, on that ground, to be regarded as a distinct species. 
In like manner, neither the two valves of a diatom, nor the stages of 
growth through which it passes, however unlike they may appear, 
should be so regarded : and, however convenient it may be to give 
distinctive names to each slight variation in diatoms, I venture to 
maintain that the practice is unscientific. 
No true conception of the limits of a species in diatoms can be 
formed until the complete life-history of these organisms has been, at 
any rate in some instances, traced out ; and this has yet to be accom- 
plished. Meanwhile it will be better to refrain from designating as 
“ species,” not only forms which, by actual observation, are known to 
appertain to an already established species, but also forms which, 
from what we know of other species, may be reasonably believed to 
belong to or spring from established species. 
