OAN S 1538 
JOURNAL 
OF THE 
ROYAL MICROSCOPICAL SOCIETY. 
DECEMBER 1897. 
TRANSACTIONS OF THE SOCIETY. 
VII. — The Limits of Species in the Diatomaceas. 
By Thomas Comber, F.L.S., F.R.M.S. 
( Bead Oct. 20th , 1897.) 
It is well known in how variable a sense the term “ species ” has 
been used by authors. It may be well, therefore, for me to explain 
at the outset the sense in which I shall use the term. 
Before the publication of the * Origin of Species/ and while the 
belief in a special creation still prevailed, the following definition was 
drawn up by a leading British botanist : * “ A species comprises all 
individual plants which resemble each other sufficiently to make us 
conclude that they are, or may have been, all descended from a 
common parent. These individuals may often differ from each other 
in many striking particulars, . . . but these particulars are such as 
experience teaches us are liable to vary in the seedlings raised from 
one individual.” 
This may, I think, be accepted as the meaning generally attached to 
the term at that time. In practice, systematists differed very widely 
in their opinions as to what forms should be included in a particular 
species ; but in theory they were pretty well agreed as to what a 
species was. 
But after the appearance of Mr. Darwin’s work, and since the 
views therein expressed have been generally accepted and further 
developed, the conception as to what a species is has become very un- 
settled. Many naturalists hold that “ it follows from the general fact 
of evolution that species are merely arbitrary divisions which present 
no deeper significance from a philosophical point of view than is pre- 
sented by well-marked varieties, out of which they are in all cases 
believed to have arisen.” f “ It is not to be anticipated that any com- 
petent naturalist will nowadays dispute that the terms ‘variety/ 
‘ species/ and ‘ genus ’ stand for merely conventional divisions, and 
that whether a form shall be ranked under one or other of them is 
* Bentham’s ‘ Handbook of the British Flora,’ 1 858. 
f Romanes, ‘Darwin, and after Darwin,’ ii. (1895) p. 60. 
2 K 
1897 
