76 ALTERNATION IN THE THALLOPHYTES 
The criterion of chromosome-number is new: the method of physiological 
experiment is also new. Still, the conclusions are in the main unaltered. 
What was then styled “homologous alternation” now stands on a basis 
of cytological unity as regards the somatic divisions, and denotes such 
recurrent phases in plants as appear to be dependent on external condition, 
not obligatory in their succession. and involve no cytological change: 
this includes the cases grouped under (1) and (2) above. There is hardly 
any need to designate such life-cycles as showing alternation at all, were 
it not that this is the type of life-history for which the term was first 
introduced by the zoologist Steenstrup. The types grouped above under 
the heading (3) were distinguished by Celakovsky as showing “ antithetic 
alternation,” and it is now found to have its basis in a cytological difference 
of the successive phases, which also show an obligatory succession, not 
determined directly by external conditions. 
The conception of normal antithetic alternation now turns upon the two 
critical points of sexual fusion and reduction: it is necessary to enquire how 
far these events are historically the same in organisms at large. It would: 
seem probable that sexual differentiation, and perhaps even sex itself 
originated along several distinct phyletic lines: on this point there is no 
definite information, though the differences of character of the organisms 
which show the simplest types of sexuality distinctly suggest that it had 
not one common source only. In the present state of uncertainty it seems 
undesirable to depart from the usual convention by which the zygote is 
held to be “homologous”; and, accordingly, it serves as a point for general 
comparison between representatives of distinct phyla. But it must be 
distinctly understood that this is in itself a conventional understanding, and 
that its adoption for convenience of description does not necessarily imply a 
strict ‘““homogeny,” in the sense that sexuality was established once for 
all. Similarly with reduction, which is theoretically a necessary consequence 
of sexual fusion, it is only by a similar conventional understanding that 
in divers organisms the cell where this is initiated is held to be “homologous” : 
it is not to be assumed that it is truly ‘‘homogenetic” in distinct phyla, 
as though reduction had been initiated once for all in sexual organisms. 
But, on the other hand, in organisms that are akin, such as the members of 
the phyla of the Ferns or the Mosses, it may reasonably be held as probable 
that the zygote and the spore-mother-cell are actually identical things, in fact 
homogenetic for the whole phylum, in the sense that each probably sprang 
from a phyletic source common for the whole phylum. 
A comparison of plants at large as regards the position of the reducing 
process in the life-cycle relative to sexual fusion shows great differences, 
as we have seen. It is not improbable that these may have been due 
in part to initial differences: we have no right to assume that there was 
uniformity at the outset. Some ground for the view that initial differences 
‘existed is to be found in such cases as the Desmids and Diatoms; for in 
Closterium the rejection of the superfluous nuclei, and probably also 
