612 Proceedings o f Royal Society of Edinburgh. [sess. 
lakes, just some half a dozen Rhizopods, and most of these merely 
varieties, closely related to well-known species. If a simple 
explanation of the origin of these abyssal Rhizopods could be 
offered which would not require us to postulate some unknown 
means whereby the peculiar species might be transferred from 
lake to lake, it would be worthy of consideration. I think we 
have such a simple explanation in the suggestion that the few 
peculiar abyssal Rhizopods found in Loch Hess are probably not 
really good species or varieties of any degree of permanence, but 
merely modifications of common littoral species, directly produced 
by the influence of the abyssal conditions on each individual 
during its period of growth. It will be observed that the majority 
of Dr Penard’s abyssal Rhizopods are varieties, and that in one 
instance it was doubtful to which of two related species — one 
littoral, the other abyssal — the variety found in Loch Hess should 
be referred. 
The term variety is used by different authors in such diverse 
senses — by some being employed as though equivalent to species, 
by others as denoting a sub-species, a race or a strain, while still 
others apply it to mere states or conditions — that in an investigation 
of this sort it is desirable to indicate what is implied by the term. 
It seems to me important to restrict the term to those forms which 
we believe to be stable, though not of specific value, and that it 
should not be applied to mere peculiar modifications which have 
no permanence. If this distinction is not made between stable 
and temporary modifications, and we are content to name as 
distinct varieties, or even as species, every peculiarity of form 
observed, the physiological significance of variation is liable to 
be obscured. 
This is well illustrated in the case of the hyaline Daphnia so 
common in lakes. This animal is exceedingly variable, and the 
extremes of form are very different in appearance from one 
another. So long as these various shapes of Daphnia were re- 
garded as distinct species no progress was made towards an 
understanding of the causes of the variation, — we had a number 
of species, that was all. When, however, it came to be recognised 
that they were all due to changes of form of one species, attention 
was devoted to discovering the causes of the variation, and now, 
