EVOLUTION OF SEX 63 



more passive egg-cells or ova ; and {b) that these two 

 different kinds of reproductive cells usually come to 

 nothing unless they combine. 



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. Here we see the formation 

 of dimorphic reproductive cells in different parts of the 

 same organism. But we may also find Volvox 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 {katabolic) processes and con- 

 structive {anabolic) processes in the protoplasmic meta- 

 bolism 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, e.g. Protomyxa, and Mycetozoa, e.g. flowers 



of tan {/Ethalium septicum). 



{b) Multiple conjugation, in which more than two cells unite and fuse 



