52 REPRODUCTION AND LIFE HISTORY. 



Evolution of sex. — A further problem is to account for 

 the two facts — (a) that most animals are either males or 

 females, the former liberating actively motile male elements 

 or spermatozoa, the latter 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. 94.) 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 (kaiabolic) 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 condi- 

 tions (such as abundant food, favourable temperature) which 



