THE EVOLUTION OF SEX. 53 



forming and usually liberating 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. Begin with those interesting organisms which are on 

 the border line between Protozoa and Metazoa, the colonial 

 Infusorians of which Volvox is a type {see p. 95). The adults 

 are balls of cells, and the component units are connected by 

 protoplasmic bridges. From such a ball of cells repro- 

 ductive 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 sper- 

 matozoa. 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 metabolism 

 varies from type to type. We believe that the contrast 

 between the sexes is another expression of this fundamental 

 alternative of variation. 



This theory may be confirmed in many ways, e.g., by 

 contrasting the characteristic products of female life, — 

 passive ova, with the characteristic products of male life, — 

 active spermatozoa ; or by comparing the complex con- 

 ditions (such as abundant food, favourable temperature) 

 which tend to produce female offspring, with the opposite 

 conditions which tend to produce males ; or by contrasting 

 the secondary sexual characters of males (e.g., bright colours 

 and smaller size), with the opposite characteristics of females. 



