so REPRODUCTION AND LIFE-HISTORY OF ANIMALS. 



In the animal series, the sexual method of liberating 

 special germ-cells has predominated, as every one is aware, 

 over the asexual method of forming buds. Moreover, the 

 sexual method has been accentuated in most animals by the 

 evolution of two sexes, which produce different kinds of 

 germ-cells — ova and spermatozoa. 



The Evolution of Sex. — Our problem now is not that of 

 accounting for the fact that most animals liberate special 

 reproductive cells, but for the two facts, {a) that most 

 animals are either males or females, the former liberating 

 actively motile male-elements or spermatozoa, while the 

 latter form and usually liberate 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. As adults they 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 conditions, 

 however, when nutrition is checked, a less direct mode of 

 reproduction occurs. Some of the cells grow at the expense 

 of their neighbours, and become large well-fed elements, or 

 ova; others, less successful in the exploitation of their 

 neighbours, divide into many little units. 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 Vo/voxhaWs in which only 

 ova are being made, and others in which only small motile 

 reproductive cells are being made. 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^ 

 between relative disruptive {liatabolic) predominance, and 

 relative constructive (anabolic) predominance in the proto- 



