38 POPULAR SCIENCE MONTHLY. 



In unicellular plants, and in some protozoans, two individuals 

 physically alike, or nearly alike, and moving about in water, unite to 

 form a single individual. In animals more complicated entire indi- 

 viduals no longer merge into one; but this function has become re- 

 stricted to certain cells, just as the function of moving the animal 

 from place to place has become restricted to certain cells. In the 

 majority of animals living in water these cells are liberated in the 

 water, and here two cells unite, as in lower forms two individuals unite. 



The cells uniting are equally important in transmitting hered- 

 itary characters, and in their essential structure. As an adaptation to 

 the necessity of the union of two cells and to the necessity that the 

 combined volume of the two cells be sufficient to give the new individual 

 a fair start, the cells, as the result of a division of labor, have become 

 very different in shape and action. One has taken upon itself the 

 function of providing the nutriment necessary to start the new indi- 



■ ■ HHP '■" 



Fig. 7. Fig. 8. 



Fig. 7. Two Typical Cells; Ovarian Eggs of Cymatogaster. 



Fig. 8. Photograph of the Conjugation of Two Strings of Individuals of Spiro- 

 gyra ; at a the contents of one individual are passing over into the next ; at b the contents of 

 two individuals are united within the wall of one ; at c the two uniting individuals have been 

 metamorphosed into a spore; d, a bachelor which missed a mate. 



vidual, and has become comparatively large and inactive. The other 

 has taken upon itself the function of providing the mobility necessary 

 to insure the union of the two cells. It has become excessively minute, 

 and very mobile. The differences in the cells concerned in sexual re- 

 production extend to the ducts through which the cells are emitted, and 

 secondarily to the whole individual, so that in every organ, function 

 and psychical trait male and female are different. 



Into the question of the advantages of the production of a new in- 

 dividual by the union of two cells we can not enter in detail. Suffice 

 it to say that on the part of some it is looked upon as the union of two 

 hereditary tendencies, which eliminates extreme badness, or what 

 amounts to the same thing, extreme goodness that may be inherent 

 in one of these tendencies, and at the same time insures new combina- 

 tions of characters from which nature may select the fit.* On the 



* Pearson has demonstrated mathematically by comparing parents with 

 offspring that ' whatever be the physiological function of sex in evolution, it is 

 not the production of greater variability.' 



