416 Evolution and Adaptation 
other kind of germ-cells. This occurs, however, only in the 
lowest forms. A step higher we find that only a part of the 
cells of the colony are set aside for purposes of reproduction. 
The cells surrounding these germ-cells may form distinct 
organs, which may show certain differences according to 
whether they contain male or female germ-cells. When 
these two kinds of cells are produced by two separate indi- 
viduals, the individuals themselves may be different in other 
parts of the body, as well as in the reproductive organs. 
When this condition is reached, we have individuals that 
we call males and females, because, although they do not 
themselves unite to form new individuals, they produce one 
or the other kind of germ-cell. It is the germ-cells alone 
that now combine to form the new individual. 
Amongst living groups of animals we find no such complete 
series of forms as exist in plants, and the transition from the 
one-celled to the many-celled forms is also more abrupt. On 
the other hand, we find an astonishing variety of ways in 
which the reproduction is accomplished, and several ways in 
which the germ-cells are carried by the sexual individuals. 
Let us examine some of the more typical conditions under 
the following headings: (1) sexes separate; (2) sexes united 
in the same individual; (3) parthenogenetic forms; (4) ex- 
ceptional methods of propagation. 
1. Sexes Separate ; Unisexual Forms.’ — Although the ani- 
mals with which we are more familiar have the sexes sepa- 
rate, this is far from being universal amongst animals and 
plants; and, in fact, can scarcely be said to be even the rule. 
When the sexes are separate they may be externally alike, 
and this is especially true for those species that do not unite, 
but set free their eggs and spermatozoa in the water, as fish, 
frogs, corals, starfish, jellyfish, and many other forms. In 
other animals there are sometimes other secondary differ- 
1 Geddes and Thompson’s “ The Evolution of Sex * has been freely used in the 
preparation of this part of this chapter. 
