SEXUAL REPRODUCTION. 195 
separation from the parental individual develops into a 
many-celled body, and in this repeats the forms and vital 
phenomena of the original producing organism. This last 
form of monogonic propagation—that of the germ cells, or 
spore-formation—leads us directly toa form of propagation 
which is the most difficult of all to explain, namely, sexual 
propagation. 
Sexual or amphigonic propagation (Amphigonia) is the 
usual method of propagation among all higher animals and 
plants. It is evident that it has only developed, at a very 
late period of the earth’s history, from non-sexual propaga- 
tion, and apparently in the first instance from the method 
of propapation by germ-cells. In the earliest periods of the 
organic history of the earth, all organisms propagated them- 
selves in a non-sexual manner, as numerous lower organisms 
still do, especially all those which are at the lowest stage of 
organization, and which, strictly speaking, can be considered 
neither as animals nor as plants, and which therefore, as 
primary creatures, or Protista, are best excluded from both 
the animal and vegetable kingdoms. In the case of the 
higher animals and plants, the increase of individuals, as a 
rule, is at present brought about in the majority of cases by 
sexual propagation. 
In all the chief forms of non-sexual propagation mentioned 
above—in fission, in the formation of buds, germ buds, and 
germ cells—the separated cell or group of cells was able by 
itself to develop into a new individual, but in the case of 
sexual propagation the cell must first be fructified by 
another generative substance. The fructifying male sperm 
must first mix with the female germ-cell (the egy) before 
the latter can develop into a new individual. These two 
