428 MR. R. GURNEY ON THE CRUSTACEAN PLANKTON 
2. Ill Barope L. m'xcrurus is only found in lakes wliicli can be proved to 
have been separated from the sea in post-Glacial times. It is therefore a 
"relict" form in the strictest sense. It has not arrived in any of the lakes 
it now inhabits by active migration. 
3. The shape of the head in L. macrurus, while not at all variable among 
individuals in any given lake, varies greatly according to the locality, and 
it can be shown that those lakes (in Europe) which contain the extreme 
fresh-water form have been separated longest from the sea, whilst in more 
recent lakes the Liinnocalanii s approaches more nearly to the grimaldi 
form. 
4. It is evident that L. grimaldu is the parent form, and that L. macrurus 
is a variant from it which has arisen independently in many places, the effect 
of the change of medium being always in the same direction. '; 
5. The first step in the transformation occurred in the Baltic during the 
'• Ancylus Lake'" period, and the Baltic lakes in which Limnocalanus occurs 
are relicts of the Ancylus lake. 
With the most interesting speculations of Ekman on the subject of the 
origin of species founded on these facts we have no concern here. 
Now, while it seems to me that Ekman's conclusions with regard to the 
Limnocalanus of the Baltic area and of the Caspian Sea are entirely well 
founded, there are certain criticisms to be made, and it is difficult to extend 
his explanation to the Limnocalanus of Ennerdale and the lakes of North 
America in whicli it occurs. 
In the first place I cannot unreservedly accept Ekman's basis of measure- 
ment of the head form on which much of his argument depends. Ekman 
has adopted the following system of measurement : — 
(1) Head-length — to the dorsal groove immediately in front of the mandi- 
bular muscle. (See text-fig. 2.) 
(2) Head-height — B — expressed as a percentage of half the head-length. 
(3) Height of vertex above or below the dorsal line of the thonix expressed 
as a percentage of half the head-length. 
The last measurement is negative in all the grimaldii forms, and positive 
only in the extreme macrurus forms where the dorsal contour of the head 
swells up above ttie dorsal line of the thorax. 
While the first and second measurements are fairly definite and extra- 
ordinarily constant, the third seems to me so uncertain as to be of little value, 
since (as Ekman has himself pointed out) the selection of a dorsal horizontal 
base-line is a matter of guess-work. I have been quite unable to find agree- 
ment between his own figures for this character and the drav/ings of the 
specimens from which the measurements were taken ; in fact, these measure- 
ments cannot, as I believe, give a real definition of the shape of the head, 
which can only be shown in a drawing. Still, for purposes of comparison, 
I give, in Table 6, measurements taken by Ekman's methods for the 
Ennerdalo and other forms. 
