50 INTRODUCTION. 
The simplest and most primitive organisms known reproduce them- 
selves asexually, and we are obliged to assume that, from a phylo- 
genetic standpoint, sexually differentiated organisms were descended 
from asexual forms. The process of this descent is clearly illustrated 
by certain of the green alge in which the sexual act consists in the 
fusion of exactly similar motile gametes. These gametes were un- 
doubtedly derived from asexual swarm spores, which they closely 
resemble, except in that they are smaller and often have fewer cilia. 
In Ulothrix, for example, and in many of the green alge, the gametes 
are, so far as is known, smaller and possess only two cilia, while the 
larger asexual swarm spores bear four cilia. Both sporangia and 
gametangia are homologous structures, and, so far as is known, the 
gametes differ only physiologically from the asexual spores. 
According to Strasburger, to use the language of the translation :* 
The sexually differentiated plants manifest certain differences in their onto- 
geny, from which it is possible to infer what was the course along which the 
phylogenetic differentiation proceeded after sexual differentiation had taken 
place. The simplest case is that in which the product of fertilization gives rise 
to an individual similar to those which gave rise to the product of fertilization, 
and which closes its own life history with the development either of sexual 
organs or of asexual organs homologous with them. This occurs in many 
Chlorophycee, where, from the zygospore (the product of the coalescence of 
similar gametes) or the odspore (the product of the coalescence of dissimilar 
spermatozoids and ova), a generation is developed which resembles the preced- 
ing and gives rise either to swarm-spores or to sexual cells homologous with 
them. Generally, any one sexual generation follows after a number of asexual 
generations, the relation being, however, dependent on external conditions, so 
that, as Klebs has shown, the development of a sexual or an asexual generation 
can be determined by the observer. In such cases there is a homogeneous 
sequence of generations which does not include any other kind of sequence or 
alternation beyond the development either of asexual reproductive organs or 
of sexual organs homologous with them. The asexual reproductive organs are 
especially concerned with the rapid multiplication of individuals under favorable 
external conditions ; whilst sexual reproduction is of importance in maintaining 
the existence of the species under circumstances which are unfavorable to the 
vegetative existence of the individual. At the same time, sexual reproduction 
ensures certain advantages arising from the coalescence of distinct sexual cells. 
In proportion as the asexual mode of reproduction was replaced by the 
sexual, the numerical conditions of multiplication were maintained either by 
the development of a number of odspores, as in certain Fucacez; or, in addi- 
tion to the sexual organs, altogether new organs were developed to ensure rapid 
and vigorous development of new individuals in an asexual manner. This 
took place in various ways. Either asexual reproductive organs were inter- 
calated in the life history of the original generation, or an altogether new 
asexual generation was developed from the product of the sexual act. 
“1? English translation, Ann. Bot., 8; 281-316. 
