W. C. Worsdell. 
i 26 
The most primitive unicellular plants were almost certainly 
asexual, the sexual character being a subsequent modification ; for 
the undifferentiated must precede the differentiated along the main 
path of evolution ; and the very earliest and most important form 
which differentiation assumes is the production of the two opposite 
yet complementary poles : the positive and negative, or male and 
female sex, by the blending of which the greatest amount of vitality 
and capacity for variation is infused into the product. 
Now, the asexual unicellular forms most probably have given 
rise to asexual filamentous organisms, and these must have been the 
earliest progenitors of all filamentous Alga; 1 with which we are 
acquainted to-day. Sooner or later, however, sex also arose in 
these filamentous and other thalloid forms; it is a noticeable fact 
that in thallus-forming plants both sexes occur as a rule on the 
same plant. 
There can be no doubt that the spermatozoid and oogonial cell 
of certain more advanced filamentous Alga;, e.g. Vaucheria, are the 
exact homologues of the free-swimming male and female gametes 
of other more primitive forms, such as Ulothrix. In any case, in 
the primitive forms the result of fertilisation was (at any rate 
frequently), the germination of the zygospore directly into a new 
filamentous plant which was asexual , inasmuch as it did not produce 
sexual organs but, on the contrary, neutral zoospores. Each of these 
latter, on germination, produced the sexual plant once again. Here 
we see a distinct alternation of generations; hut it is an homologous 
alternation of generations, viz., between a neutral or gonidial and a 
sexual or oogonial generation respectively ; the two generations are 
homologous because both exhibit the same mode and habit of growth in 
every respect, they are precisely the same plants differing only in the 
character of their reproductive organs. Examples are afforded by 
Ulothrix, Vaucheria, Dictyota, &c. amongst the Algae. A very 
interesting example of this method of alternation is found in the 
Moss, where the primitive asexual filamentous protonema gives rise 
(herein exhibiting a striking adaptation to its acquired land-habitat) 
by vegetative budding, and no longer by zoospores, to the leafy 
Moss-plant. 2 Yet another instance of this homologous alternation 
of generations is to be observed in the higher Fungi and Florideas; 
in the former the ascus and basidium and in the latter the carpo- 
1 I use this term to include also all primitive Fungi. 
2 Here the two generations differ altogether in habit and mode of 
growth ; nevertheless they are clearly equivalent to the two 
generations constituting the homologous alternation in the 
Alga:. 
