232 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
in any figures except one by Nitardy (714, pl. 5. f. 4), which 
I have reproduced (ғіс. 27d). It would seem that the tendency 
to lobing which fits so perfectly with the principles of least sur- 
faces and binary fission in the simpler forms of the diactinial 
type has here gone too far and become distinctly non-adaptive 
as far as the symmetrical grouping of the cells is concerned. 
The tendency to the lower numbers of cells in the colonies of 
these species is, if real, a curious correlation, since the quadrifid 
form of the cells would not in any case seem capable of limiting 
the number of times they should divide in reproduction. 
DiscussioN 
The general relations of the integrum, Monactinium, Diac- 
tinium, Triactinium and Tetractinium types suggest at once 
certain evolutionary possibilities and limitations in very clear 
form owing to the extreme simplicity of the characters involved. 
Evolution in the whole group has proceeded by modification of 
the cell form. It is quite obvious, as noted, that a species with 
one spine could not become gradually modified into a species 
with two spines in any other way than by returning to the spine- 
less type and then advancing on quite a different line of develop- 
ment resulting in typically different intercellular relations in the 
colony. We have no evidence of the possibility of transforming 
a simplex type into a two-spined type by the gradual development 
of a second spine in addition to the one already present or by 
splitting the single spine. A form with one long, well-developed 
spine and one short, rudimentary spine is not only unknown in 
nature but is quite inconsistent with the colonial organization 
of the cells in the plate-shaped groups which are characteristic 
of the whole genus. The only obvious evolutionary routes from 
a one-spined to a two-spined type are either as noted by a change 
back to the primitive integrum type and a new start in a character- 
istically different direction or by sudden mutational transforma- 
tion, perhaps to be considered a reduplication, by which a form 
with one spine becomes at once a form with two equally well- 
developed spines. There isan analogy here with the reduplications 
in the lobing of fern fronds and pinnae by which the common 
sports of the Boston fern have been produced (see Benedict, 716). 
