THE EVOLUTION OF SEX 31 



BODY-CELLS AND GERM-CELLS. The next 

 step in the evolution of sex was the confirming 

 of what forms like Volvox began the setting 

 apart of special reproductive cells as dis- 

 tinguished from the somatic cells. In a de- 

 veloping embryo we see a process of division 

 of labour, whose structural result is called 

 differentiation. Some cells become contractile 

 and some sensitive, some digestive and others 

 skeletal, and so on, but some do not share at 

 all in body-making and these are the germ- 

 cells that continue ihe race. 



What we see beginning in Volvox is 

 established in almost all multicellular organ- 

 isms, from sponge and polyp to bird and 

 mammal, from seaweed and liverwort to 

 fern and tree namely, sexual reproduction, 

 the continuance of life by means of special 

 germ-cells. In many cases it is combined 

 with asexual reproduction by buds or frag- 

 ments, and the difference between the two 

 methods must be carefully considered. 



ADVANTAGES OF SEXUAL REPRODUCTION. 

 The question thus arises : What are the advan- 

 tages of the liberation of special germ-cells as 

 against the process of budding, or of frag- 

 mentation where a considerable sample of the 

 body is set adrift ? (a) An obvious answer is 

 that the method of sexual reproduction 

 starting a new organism from a fertilised 

 ovum is much more economical than the 

 asexual process. In the one case two cells 

 suffice to start a new organism; in the other 

 cases thousands of cells may be required. 

 There is something in this answer, but it 

 cannot be the complete one, since many plants 



