Sex as an Adaptation 415 
method of sexual reproduction. Two amcebas, or ameceba- 
like bodies, thus flow together, as it were, to produce a new 
individual. 
In the more highly specialized unicellular animals, the 
processes ate 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 
paramoecium, 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 algee 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 
