LIMNOLOGICAL PROBLEMS 
409 
Just because the new direction in planktology has to such a high 
degree given naturahsts the opportunity of observing how variable 
the conception of species within this sphere of work is, it is difficult 
to understand the demand of the older systematists, that their view 
of the species must be the only right one. 
make. On p. 73 of my plankton work I have expres^secl myself very carefully. 
I say : " I still believe it very probable that the ' species ' are stages in a variation 
series ; but whether these stages are fixed species in the sense that the variation 
series consists of a number of temporary species following each other, or whether 
the transformations are only connected with certain generations and broods of 
the same species, we do not at present know." I have thus stated expressly that 
I was unable to determine with regard to the species mentioned whether they 
were true species or only seasonal forms ; and I pointed out also that the material 
on which my account was based had been procured by others (M. Voigt), Nothing 
of this is mentioned by Rousselet in his criticism. 
Kousselet maintains that the species mentioned cannot be seasonal variations 
since, according to his statement, they are all found at the same time. This 
objection is not of any importance, as I have pointed out expressly on p. 251. 
This will naturally be the case with the seasonal variations at most times of the 
year. The one form does not disappear on the same day that the other arrives ; 
but whilst the one is decreasing numerically the other is increasing — a fact which 
will be observed by anyone who follows regularly the forms throughout the year. 
Stragglers or relicts there must always be of all the numerous forms : I have even 
found S. pecti?mta and tremula side by side in winter. What interests me most 
in this matter is, has Rousselet at anytime found 'SS. grartdis^^ in the months 
of February and March ? With Rousselet's statement that " the periodic and often 
very sudden disappearance and reappearance of various Rotifers is a well-known 
fact, and the Syn elite ta follow the same habits," I am naturally in agreement. 
But the fundamental matter here, just this "sudden disaj^pearance and reappear- 
ance," is not apparently for Rousselet a problem which requires a solution. Now I 
have endeavoured to give a solution to just this problem by regarding the Synchseta 
as species, in Rousselet's sense if preferred, but vicarious seasonally, with different 
times for breaking up the resting-stages and different requirements as to tempera- 
ture. But Rousselet seems to have quite overlooked this or misunderstood it. 
Further evidence that the above-named Synch?e,ta species are in reality species 
and not seasonal forms is found by Rousselet in this, that " of all plankton 
Rotifers the Syncliaita are the most vigorous swimmers, and quite able to 
counteract by their cilia any slight tendency to sink that may be due to a decrease 
in the density and viscosity of the water in summer." To this I would remark, 
in the first place, that if Rousselet had ever seen a Ploesonia Hiidsoni swimming 
he would scarcely have taken the Synchseta to be the most vigorous swimmers ; 
but in the second place, and more especially, whence can Rousselet obtain even 
the remotest evidence for the view that the wheel-organ of the Synchseta is able 
to counteract the fluctuations in the density and viscosity 1 Here, as not so 
seldom elsewhere, we find the "exact" systematist, without even a shadow of 
scientific evidence, throwing out random postulates and requiring them to be 
believed. At the same time he subjects to a superficial and one-sided criticism 
the views of naturalists who for years have studied the phenomena on which 
the views rest out in nature itself, under conditions with which the systematist 
has no acquaintance, and even though these views are put forward in an exceed- 
ingly cautious form. 
