174 THE POPULAR SCIENCE MONTHLY. 



tially in the union or conjugation of two diverse cells (sperm and germ 

 cell) to produce one cell (ovum). Now, in the lowest forms of sexual 

 reproduction, among unicelled organisms, the conjugating cells are not 

 2')erceptihly different ; so that the element of diversity in the conju- 

 gating cells may be eliminated from the essential conditions of this 

 mode. In parthenogenetic reproduction of female offspring, as in the 

 case of moths and phyllopods, we have the other element, i. e., the 

 necessity of tioo cells, eliminated ; so that there remains nothing which 

 is absolutely essential. 



(c.) Facts which bear on the First Step, viz., Derivation of Non- 

 Sexual Modes from Ordinary Processes of Growth. — The transition 

 between the lowest form of non-sexual reproduction, viz., fission, and 

 ordinary growth, is so obvious that it is hardly necessary to insist on 

 it. A single cell divides itself into two ; each half grows, and again 

 divides itself into two, and so on. Now, if the cells cohere, we call it 

 growth ; if they separate, we call it reproduction. Again : a mass of 

 cells grows by continued cell-multiplication, as above. Finally, the 

 increasing mass or community becomes too large to be managed well 

 from one center ; it therefore divides itself into two masses or com- 

 munities, each of which continues to grow as before. It is plain that 

 a slight difference only in the degree of cohesion determines whether 

 the same process be called growth or reproduction. 



Thus we have shown the easy gradation, and therefore the prob- 

 able derivation, of the highest mode of sexual reproduction — the uni- 

 sexual — from the ordinary processes of growth, through the different 

 grades of asexual and bisexual reproduction. The derivation of dif- 

 ferent modes of sexual reproduction from each other will not, I think, 

 be questioned. Still clearer is the fact that non-sexual reproduction 

 is but a modification of the ordinary process of growth. The only 

 place where there is any gap is between the asexual and the sexual 

 modes. Throughout growth and non-sexual modes of reproduction we 

 have everywhere only cell-nndtiplication — everywhere we have divis- 

 ion of one to form two: in sexual reproduction, on the other hand, we 

 have the contrary process, viz., tJie Pinion of tico to form one. Yet 

 this gap is certainly partly filled by the larval reproduction of aphids, 

 by those cases of parthenogenesis in which unfertilized ovules pro- 

 duce females, and those cases of true sexual generation in which the 

 conjugating cells are similar. 



5. Outline of Probable Histoet of the Process. — The grad- 

 ual evolution of the higher forms of sexual reproduction probably took 

 several different roads. There is little doubt that in some cases sexual 

 reproduction in its simplest form was reached at a very early period. 

 It is probable, for example, that in very early times unicelled organisms 

 multiplying usually by fission (asexual) from time to time conjugated 

 (sexual). The simple form of sexual reproduction thus reached was 

 afterward perfected. But it is also probable, nay, judging from the 



