MESOTENIUM, AND SPIROTANIA. 221 
Micrasterias rotata and M. denticulata—Euastrum didelta 
and E£. ansatum—as in each case but varieties of a single 
species, &. Why admit certain denticulations, and incisions, 
and processes, and lobes, in these forms to be good specific 
marks, and then arbitrarily stop short, and disallow other 
characters of the same nature possessed by one of the dis- 
puted forms, and not by the other, and which each refuses to 
lend to the other, and say they are of no value—although, so 
far as we know, the species depending on them can be recog- 
nised wherever the two forms are found in various countries 
of Europe and in Bengal ? 
Dr. Wallich believes that “such differences indicate mere 
accidental varieties, handed down, no doubt, from parents to 
progeny in the same locality, so long as physical conditions 
remain the same.” If certain external physical conditions 
be the cause of such minor individual characters, and if dis- 
similar conditions will cause their obliteration or transference, 
how is it that, under all conditions in which Micrasterias 
rotata and M. denticulata (for instance) present themselves, 
they maintain, at least so far as we know, their own ultimate 
characters? With us here they are both about equally 
common in their own localities. It is clear that the greater 
number of subdivisions of the former, its larger middle lobe, 
its more acute teeth, its greater size, &c., give it no advantage 
over the latter in the “ struggle for life,” although both have 
the preponderance in numbers (in whatever the advantage 
may consist) over certain other well-marked allied forms. I 
think it seems to follow, from Dr. Wallich’s statement of his 
views, that “natural selection” must in his opinion fall into 
the background so far as these organisms are concerned ; for, 
according to him, characters derived from parents, however 
seemingly inherent here, must at once succumb to varying 
surrounding physical conditions. 
Dr. Wallich says that the onus probandi, as regards that 
side of the question against which I contend, does not lie 
with those who think with him; but “ that it is sufficient to 
show a fair number of cases (as, for instance, in the genus 
Micrasterias) in which unquestionable interchange of those 
characters is to be met with, which by Ralfs and others have 
been seized upon as indicative of a distinct origin.” Dr. 
Wallich will, I hope, excuse me if I still hold that such cases 
have not yet been shown in the established species of 
Micrasterias ; and that those “interchanges of characters” 
are founded upon assumption of what it is presumed might 
be, rather than what is. I venture to hold still.that the 
interchange of characters between the various species of 
