Sex as an Adaptation 415 



method of sexual reproduction. Two amcebas, or amceba- 

 like bodies, thus flow together, as it were, to produce a new 

 individual. 



In the more highly specialized unicellular animals, the 

 processes are different. Thus in vorticella, a small, active 

 individual unites with a larger fixed individual. The proto- 

 plasm fuses into a common mass, and a very complicated 

 series of changes is passed through by the nucleus. In 

 paramcecium, a free-swimming form very much like vorti- 

 cella, two individuals that are alike unite only temporarily, 

 and after an interchange of nuclear material they separate. 



In the lower plants, and more especially in some of the 

 simple aggregates or colonial forms, there are found a num- 

 ber of stages between species in which the uniting individuals 

 are alike, and those in which they are different. There are 

 several species whose individuals appear to be exactly alike ; 

 and other species in which the only apparent difference 

 between the individuals that fuse together is one of size; 

 and still other species in which there are larger resting or 

 passive individuals, and smaller active individuals that unite 

 with the larger ones. In several of the higher groups, in- 

 cluding the green algae and seaweeds, we find similar series, 

 which give evidence of having arisen independently of each 

 other. If we are really justified in arranging the members 

 of these groups in series, beginning with the simpler cases 

 and ending with those showing a complete differentiation 

 into two kinds of germ-cells, we seem to get some light as to 

 the way in which the change has come about. It should not 

 be forgotten, however, that it does not follow because we can 

 arrange such a series without any large gaps in its con- 

 tinuity, that the more complex conditions have been gradu- 

 ally formed in exactly this way from the simplest conditions. 

 So far we have spoken mainly of those cases in which the 

 forms are unicellular, or of many-celled species in which all 

 the cells of the individual resolve themselves into one or the 



