XVI.] THE LAWS OF VARIATION. 191 



a single cell, cells divide into cells ; and when tliey have 

 divided, they separate. Among forms a little higher, they 

 adhere together after dividing, and constitute cellular tissue. 

 When they separate, we call the result propagation ; when 

 they adhere, we call it growth : but there is evidently no 

 fundamental difference between tlie two cases ; they are 

 found in nearly allied forms, and, indeed, they graduate 

 into each other through species in which the adhesion of 

 the cells is very slight.^ 



The reproduction just described is non-sexual ; but the simplest 

 first and simplest form of sexual reproduction is presented ^°''"^ "*^ 



^ . ^ '^ sexual 



by those same unicellular Alga. There is no distinction reproduc- 

 of sex, but reproduction takes place by means of the fusion unicellular 

 into one mass of the germinal matter that forms the con- Algse. 

 tents of two cells. Among the Diatomacese and Desmideae, 

 two individuals, each of them consisting of but a single 

 cell, place themselves together and burst : the contents of 

 the two mix, and from their union a fresh brood arises.^ 

 Although there is here no visible distinction of sex, yet Its esscn- 

 what appears to be the essential condition of sexual repro- j^io*^u^s 

 duction is fulfilled ; namely, the fusion of the germinal mixture of 

 matter derived from two different organisms, or, at least, matter" 

 two different cells. In the case just described, the cell- ^1'°"^ t^'o 

 walls, after they burst and liberate their contents, are as 

 useless and dead as the cast skin of a snake. But in the 

 zygne7na — an Alga of rather higher organization, which Zygne- 

 consists of cells united into filaments, somewhat like strung "i''^ V f™" 



o piest lorm 



beads — we meet with the first and simplest form of the of sexual 

 distinction between the sexes. Two of the filaments — -that tion."*^' 

 is to say, two distinct plants — approach and lay themselves 

 alongside each other, and the contents of the cells of one 

 filament pass over into the cells of the other, where the 

 union is formed which is to give origin to a new brood.^ 

 The essential matter in sexual generation is the union of Purpose 

 germinal matter from two distinct sources ; and the pri- ^I- ^^^'^'^^^ 

 mary purpose of the distinction between the sexes -appears tion, to in- 

 to be to increase this difference. It would be premature difference 



1 Spencer's Principles of Biology, vol. ii. pp. 15, 16. 



^ Carpenter's Comparative Physiology, p. 878. ^ ji^j^j p ggj_ 



