126 Life and Death, Heredity and Evolution 



two of these can mate, just as apparently any two indi- 

 viduals can mate in Paramecium. This is particularly the 

 case with many lower plants (Figure 37). It is true even 

 in many of the moulds, in some species of which, as we have 

 before seen, physiological sex differences exist, even though 

 there is no visible difference between the sexes. In other 

 species of moulds, no such physiological sex difference exists ; 

 a given individual can mate with any other individual of the 

 species. 



Figure 37. Mating of similar cells, from the alga Stephanosphaera; 

 successive stages from left to right. The two unite to form a single 

 reproductive body or zygote (at the right). After Hieronymus, from 

 Doflein, 1911. 



From such cases, common in lower organisms, it is natural 

 to draw the conclusion, which many have drawn, that in 

 these lowest creatures there is no difference of sex ; that sex 

 has arisen as evolution progressed; that therefore sex is 

 not coextensive with and fundamental to life. 



On the other hand, some have held that the very fact that 

 two unite is sufficient proof that they are sexually diverse; 

 this for example is the view taken by Minchin (1912) in 

 his Introduction to the Study of the Protozoa and by Coul- 

 ter in his interesting work on the Development of Sex in 

 Plants (1914). 2 If this argument is advanced as one of 



9 See Minchin, p. 160: "The fact that gametes and pronuclei tend to 

 unite proves that in all cases there must he intrinsic differences be- 

 tween them which stimulate them to do so." Coulter says on page 26: 



"The gametes are alike in appearance, but that they are not alike in 

 fact is evidenced by their pairing and mutual attraction." A similar 

 idea is expressed by Collin (1909): "But this invisible difference be- 



