152 THE EVOLUTION OF SEX. 



as we have seen, very generally in the Protozoa, and is also a 

 common fact in the life-history of simple Alg^. It is open to 

 every one possessed of a microscope to observe what conjuga- 

 tion means in such a common fresh-water alga as Spirogyra. 

 Opposite cells of adjacent filaments are attracted to one another 

 by what a recent observer calls a "purely physical process," and 

 the contents of the one cell pass bodily over into the other. 

 In the great majority of cases where conjugation occurs, the 

 uniting cells are to all appearance similar, l^ut it must be 

 remembered that it does not follow from this that they are 

 physiologically alike (see fig. p. 143). 



"^3 



Diagrammatic representation of the contrast 

 between conjugation (horizontal line) and 

 fertilisation (vertical line). 



id) Both among plants and animals, all naturalists are 

 agreed that it is impossible to draw any line between the con- 

 jugation of similar and the union of more or less dimorphic 

 elements. "This differentiation presents," Sachs says, "especi- 

 ally in Algse, a most complete series of gradations between the 

 conjugation of similar cells and the fertilisation of oospheres by 

 antherozoids, any boundary line between these two processes 

 being unnatural and artificial." The gradual appearance of 

 dimorphism has been already noted in discussing the origin 

 of sex, and need not be re-emphasised. 



\e) Lastly, in fertilisation among higher plants and animals, 

 the two elements which unite are highly differentiated, alike in 

 contrast to one another and in opposition to the general cells 

 of the body. A consideration of the phenomena 'm. loose pro- 

 tist colonies like Volvox or Ampullina, which suggest the bridge 



