On the Nature of Fertilization and Sex. 173 
swimming male cell. In the Algae, therefore, both male and female 
gametes may be regarded phylogenetically as special kinds of 
spores: a complete series occurs in which the protoplasm in 
the species becomes more and more highly differentiated into 
two sorts—greater differentiation being accompanied by more and 
more dependence upon fusion before the reproductive bodies can 
carry on development. The case is, in fact, comparable to that of 
a series of hemiparasites which show more and more dependence 
upon a suitable host, or to a symbiotic partnership in which the 
partners show greater and greater dependence upon one another, 
together with an increasing degree of mutual adaptation. 
It may be pointed out that although the egg-cell is usually 
spoken of as requiring the stimulus of fertilization by the male 
cell before it can develop, it would be just as true to say that the 
spermatozoid requires an egg-cell which it can enter (“ para¬ 
sitize ”) before it (the spermatozoid) can develop. Regarded from 
this latter point of view, the phenomenon occurring in fertilization 
is analogous to that observed when an obligate parasite requires 
an appropriate host before it can carry on its growth. 
What causes have been responsible for the differentiation of 
protoplasm into two kinds (male and female) in the course of 
evolution—such as appears to have occured for example in the 
genus Chlamydomonas —it is not yet possible to say ; but one may 
evidently regard the sex differentiation of the individuals which 
behave as gametes in a single species such as C. Braunii, or the 
differentiation of the gametes in any of the higher plants during 
the lifetime of the individual 1 (for both of which examples the 
same kind of forces are evidently responsible) as special cases in 
which the evolutionary history of the race plays a part in the 
ontogeny of the individual. 
The evolution of sexuality with its accompanying phenomenon 
of fertilization may be regarded, therefore, as the evolution of 
mutual parasitism within the species: and, indeed, since the 
difference involved in the differentiation of the two kinds of pro¬ 
toplasm distinguishing the sexes of a species would appear to be 
less than that involved in the differentiation of one species from 
another, one is tempted to alter the form of the above statement 
and to suggest that the evolution of sexuality may have been the 
first step towards the evolution of parasitism. 
1 In comparing a unicellular organism with one that is multicellular, it 
must be remembered that the cells which result from vegetative division 
remain united to form the “ individual ” in the latter case, while in the former 
each cell becomes a free “individual.” 
