442 MR. R. GURNEY ON THE CRUSTACEAN PLANKTON 
In a plankton sample of Sept. 1912 a single specimen was met with which 
obviously belongs to D. galeata (text-fig. 3, C). I do not think that this 
specimen can be regarded as a sport or mutation from D. hyalina, but rather 
as an instance of the transport of an ephippium from Coniston or Winder- 
mere. If this be so it goes to show that these lakes are not so isolated as 
would appear, and that interchange of species is not infrequent. Tf, in spite 
of such interchange, the Daphnias inhabiting the different lakes remain 
distinct, it is evident that the lociil races are established by right of survival, 
and it leads one to suppose that, difficult as they may be to separate, yet tliey 
may represent fixed species. Our conception of species is determined mainly 
by onr ability to discriminate individuals by fixed and measurable standards ; 
but not only are physiological differences largely ignored or are unmeasurable, 
but two races separated only by minimal mean differences may be just as 
real specific units as those distinguished by obvious characters in all indi- 
viduals. The evidence goes to show that a single lake as a rule is inhabited 
by one race of limnetic Daphnia onlj-, and, although within such a race 
extreme forms are commonly found, it is obviously of no use to give these 
variants names, as has frequently been done in the past. But, with that 
exception, it seems advisable in the present state of knowledge to recognize 
all well-marked races as distinct rather than to treat all as variants of one 
collective species. The question of real specific distinction cannot, I think, 
be determined by laboratory experiment, neither is the fact that the winter 
forms of a number of Daphnias can be shown to be morphologically 
identical prove anything more than close relationship. If the many 
forms of Daplinia are to be regarded as variants of one species originating 
imder the influence of external conditions, the reappearance in widelv- 
separated areas and under conditions which can hardly be imagined to be 
identical of precisely the same form — e.g. J>. galeata in Windermere and in 
Central Asia — is difficult or impossible to understand. It must be admitted 
that the whole problem of specific validity is raised by the genus Daphnia in 
its aculest form, and is far from being solved. 
Daphnia lacustris, Sars. 
1. Esthwaite and Hawes Water. 
The Daphnias of Esthwaite illustrate remarkably well the impossibility of 
drawing any definite distinction between the various forms now groujit-d 
within the species D. longispina. While some individuals taken in Esthwaite 
in July 1921 were indistinguishable from T>. longispina forma rosea, having 
the crest interrupted over the eye and the general form o£ that species, the 
usual type of Daplinia present in that and in other collections belongs to 
D. hyalina var. lacustris. I was at first inclined to suppose that two distinct 
races were present, but the two forms grade so completely one into the other 
that they must be regarded as one. The outline of the head may be 
completely rouoded, or may show a scarcely noticeable dorsal angle, while in 
