344 
Bower . — A Theory of the 
take place where vegetative tissues are practically absent. 
On comparative grounds we are therefore justified in con- 
cluding that the vegetative phase of the sporophyte has, in 
the course of evolution, been intercalated between fertilization 
and spore-production ; accordingly, in point of evolutionary 
history, we should designate spore-bearing tissues of the 
sporophyte as primary , while vegetative tissues of the sporo- 
phyte would be regarded as secondary. 
It will be our main object, by comparative study, to obtain 
a connected idea of some of the methods of progress of the 
sporophyte from its simpler to its more complex types. And 
here it must be expressly stated that the primary intention 
is not to trace actual homologies, or to demonstrate the 
phylogenetic relationships of known families of plants ; we 
shall rather endeavour to arrive, on grounds of detailed 
examination and comparison, together with consideration of 
the biological position of the organisms themselves, at some 
reasonable view of the methods of advance of archegoniate 
plants. 
We see on comparison of the simpler types, before any com- 
plications of external form, evidence of two chief factors of 
progress: viz. (i) increase of spore-production, and (2) sterili- 
zation of some of the potentially sporogenous cells. It will be 
shown that these two factors make their effects apparent 
among the more complex as well as among simpler forms. 
Taking first the increase of spore-production, let us consider 
it from the biological aspect: the advantage which it brings 
appears to be that by this means one sexual process is 
sufficient to produce a more numerous progeny ; in arche- 
goniate plants we are dealing with organisms probably derived 
from some aquatic Algal ancestors in which the zygote 
germinated directly, or after few divisions, to form new sexual 
plants ; but the typical Archegoniatae are land-plants ; they 
still depend, however, upon external fluid water for the 
movement of their spermatozoids, and thus fertilization is 
only possible at intervals when fluid water is present. I 
believe that we see in the extensive subdivision of the 
