EVOLUTION OF SEX, 55 
different kinds of reproductive-cells usually come to nothing 
unless they combine. “te Bs 
The problem is partly solved by a clear statement of the 
facts. Let us begin with those interesting organisms which 
are on the border line between Protozoa and Metazoa, 
the colonial Infusorians, of which Volvox is a type. The 
adults are balls of cells, and the component units are con- 
nected by protoplasmic bridges. From’such a ball of cells 
reproductive units are sometimes set adrift, and these divide 
to form other individuals without more ado. In other con- 
ditions, however, when nutrition is checked, a less direct 
mode of reproduction occurs. Some of the cells become 
large, well-fed elements, or ova; others, less successful, 
divide into many minute units or spermatozoa. The large 
cells are fertilised by the small. - Hete we see the formation 
of dimorphic reproductive cells in different parts of the 
same organism. But we may also find Volvex balls in 
which only ova are being made, and others, with only 
spermatozoa. The former seem to be more vegetative and 
nutritive than the latter; we call them female and male 
organisms respectively ; we are at the foundation of the 
differences between the two sexes. 
All through the animal series, from active Infusorians and 
passive Gregarines to feverish Birds and more sluggish 
Reptiles, we read antitheses between activity and passivity, 
between lavish expenditure of energy and a habit of storing. 
The ratio between disruptive (Aafabolic) processes and con- 
structive (azabolic) processes in the protoplasmic metabolism 
varies from type. to type. It may be that the contrast 
between the sexes is another expression of this fundamental 
alternative of variation. 
Stages in the history of fertilisation. —While it is not difficult 
to see the advantage of fertilisation as a process which helps to sustain 
the standard or average of a species and as a source of new variations, 
we can at present do little more than indicate various forms in which 
the process occurs, 
(a) Formation of Plasmodia, the flowing together of numerous feeble 
cells, as seen in the life-history of those very simple Protozoa 
called Proteomyxa, ¢.¢. Protomyxa, and Mycetozoa, ¢.g. flowers 
of tan (4 thaliwm septicum), . 
(8) Multiple conjugation, in which more than two cells unite and fuse 
together temporarily, as in some Sporozoa and in the sun- 
animalcule.(Actinospherium). a wa 
