H2 



THE EVOLUTION OF SEX. 



chanical breakage of a form like Schizogenes, so the very beginning of 

 iertilization is found in the almost mechanical flowing together of 

 exhausted cells. 



{U) Between this and the process usually described as conjugation, 

 there are some interesting links. Sometimes as many as three or 

 four spores of lowly Algce club together, as if to gather sufficient 

 momentum to make a combined start in life. The young forms of 

 the sun-animalcule (Actinosphoerium) usually unites in twos, but 

 Gabriel has observed in some cases a multiple union. So in gre- 

 garines (common parasites in invertebrates), while the usual union is 

 certainly dual, Gruber has again observed what may be termed multi- 

 ple conjugation. Union of three has also been observed as an 

 exception in several infusorians. The union of more than two may 

 thus be interpreted as intermediate between the formation of plasmodia 

 and the normal dual conjugation. 



(c) Conjugation of two similar unicellular organisms occurs, as we 

 have seen, very generally in the Protozoa, and is also a common fact 



in the life-history of simple Algce. 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 ob- 

 server calls a ' ' purely physical pro- 

 cess," 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, but it 

 must be remembered that it does not 

 follow from this that they are physiologically alike (see fig. p. 135). 



id) Both among plants and animals, all naturalists are agreed that 

 it is impossible to draw any line between the conjugation of similar 

 and the union of more or less dimorphic elements. " This differen- 

 tiation presents," Sachs says, "especially in Algce, a most complete 

 series of gradations between the conjugation of similar cells and the 

 fertilization 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-emphasized. 



(e) Lastly, in fertilization among higher plants and animals, 

 the two elements which unite are highly differentiated, alike in con- 



FlG. 48. — Diagrammatic representation of the 

 contrast between conjugation (horizon- 

 tal line) and fertilization (vertical line). 



