224 ARCHER, ON CYLINDROCYSTIS, ETC. 
less constant under the same conditions—that is, in a given 
locality—afford evidence of persistence of type. On the 
other hand, I hold they cannot be accepted as evidence of 
this persistence unless they can be proved to occur under 
every variety of conditions—that is, in widely remote locali- 
ties. I speak from experience, when I say that many—very 
many—of the assumed species of Protophyta and Protozoa 
are identical—the distinction on which their separation has 
heretofore been based being entirely the result of the acci- 
dental conditions under which they have been reared. 
“In the Desmidiacez, to which you direct attention more 
particularly, it appears to me that such differences as the 
number of indentations, the acuteness or obtuseness of the 
teeth, the number of spinous processes, and so forth, indicate 
mere accidental variations, handed down, no doubt, from 
parent to progeny in the same locality so long as the physical 
conditions remain the same; but nevertheless not to be 
regarded as constant, or as impressed on the organisms 
ab initio as an integral feature in their physiological con- 
stitution. 
“It should be borne in mind that the onus probandi does 
not rest in every example on those who think with me, but 
that it is quite sufficient that we show a fair number of cases 
(as, for instance, in the genus Micrasterias), in which un- 
questionable interchange of those characters is to be met 
with, which by Ralfs and others have been seized upon as 
indicative of distinct origin. For such cases prove that the 
law which it is assumed governs the limits of species is no 
law, but only a conditional direction, holding good only so 
long as the surrounding conditions continue the same. 
“Tf, however, the object in view in defining varieties 
under specific designations is merely to render the identifica- 
tion of similar forms more easy, I have nothing to say against 
it beyond this, that I should be loth to have to make up the 
lists even as they stand now, and firmly believe it will be an 
impossibility for the coming generation of naturalists to do 
so at all.” 
