homologous Alternation of Generations in Plants . 365 
goniatae. I cannot here do more than suggest to those who 
make these organisms their more special study, that in 
alternations of external conditions of temperature, light, 
exposure to air, varying supply of nutrition either as regards 
quantity or quality (heteroecism), the circumstances may 
sooner or later be recognised which led to the interpolation 
of a new phase in these plants. It is to be remarked that the 
Thallophytes as a whole appear to be more directly affected 
by external circumstances than the higher forms : the here- 
ditary stamp 1 seems to be less rigidly fixed upon them than 
upon the main archegoniate series : naturally this will greatly 
increase the difficulty of their comparative treatment, and 
should make us all the less ready to subject them to forcible 
comparison with the latter. I have above pointed out that 
there is in the sexual Thallophytes only one fixed comparable 
point — the zygote : and it is clearly to be understood that even 
the recognition of that as a fixed point depends upon an 
assumption, viz. that sexuality is a process uniform in its origin 
throughout sexual plants: in this we are .at present justified. 
Reasons have been above given for not recognising the spore 
as an alternate fixed point for all sexual plants, and therefore 
for dissenting from the stiff views of alternation propounded by 
Sachs. Alternation is, like other phenomena of organic life, 
to be looked upon as a result of adaptation, not in any sense 
a matter of necessity : the external conditions to which plants 
are exposed are not, and have not been all uniform, and, 
therefore, if we admit that alternation is a result of adaptation, 
we have no right to assume uniformity in type of alternation 
throughout the whole vegetable kingdom. 
I have dwelt at some length on the marked character of 
the antithetic alternation as seen in the archegoniate series of 
plants, because it is the most prominent case of alternation to 
1 It is quite apart from the object of this paper to discuss whether hereditary 
characters be the result of accumulation of the effects of external circumstances 
upon successive individuals, or of the mere selection of the favourable peculiarities 
of individuals of a variable race : the expressions used are not intended to convey 
any view on this question one way or the other. 
