174 Transactions of the Royal Microscopical Society. 
Now the report of the discussion affirms that “ Mr. Ingpen 
suggested that, after all, the question of genus and species depended 
upon difference in shape, and not upon the absolute number or 
fineness of the lines.”* From which I should gather that the 
identity of ultimate structure which I pointed out, taken together 
with the opinion of the authorities named, had left the speaker 
unconvinced of the identity of the forms in question; since 
there were palpable though minute differences of shape in the 
drawings submitted. Now I have not the slightest desire to 
defend or seek to sustain the identity. This is in far better hands; 
and the facts I presented remain the same, whether the three 
forms be identical or not. But the position taken by Mr. Ingpen 
involves a biological question of large importance. Every possessor 
of a microscope knows that if there be one thing that is permanent 
in Navicula and Pleurosigma, it is not necessarily the tenuity or 
coarseness of the striation, but its arrangement; and it was to 
this that I specially called attention in the forms in question. On 
the other hand, I am prepared amply to prove that the “ shape” 
of these diatoms is subject to almost infinite variation: small in 
any given case, doubtless, but occurring in every conceivable 
direction. Nor am I alone in this affirmation. I number amongst 
my friends and acquaintances many amateur, and several profes- 
sional, mounters of diatoms; through whose hands, and under 
whose eyes, millions of certain forms pass every year; and with- 
out a single exception they assure me that between any two con- 
trasted and comparatively unlike forms of the “same species” they 
can find almost every intermediate grade or link. 
Now the point in question is, are we to establish a species upon 
a variable or an invariable characteristic ? Is specific identifica- 
tion to depend upon that which is always changing in indefinite 
ways, or upon that which practically never changes ? 
My first great lesson in biology was learned, incidentally, many 
years ago, from Professor W. K. Parker, who dispelled for ever my 
hard-and-fast view of species in Foraminifera by showing me every 
conceivable link between two species. The meaning of it never left 
me; and supplied by him with a rich store of foraminiferal material 
from almost every part of the world, I quietly worked over hun- 
dreds of thousands of forms; and, so far as the mere shell was con- 
cerned, proved to myself clearly that many so-called species were 
merely extreme modifications of the “species ” to their right or left, 
and that every intermediate modification could be found; a truth 
which has had ample illustration from Professor Rupert Jones in 
our recent ‘ Transactions.’ + 
But years of study have convinced me further, that this 
mutability is not confined to this group of the Protozoa. It is 
* (M.M. J.’ January 1, 1877, pp. 52, 53. + ‘M.M. J.’ vol. xy. 61. 
