PRINCIPLES OF BIOLOGY. 
41 
common form of genesis; in the great majority of plants 
and in numerous animals like produces unlike, the offspring 
does not resemble the parent. This is the exact reverse of 
the opinion which the ancients held and the unscientific of 
the present day would express, but it is nevertheless the 
correct one, and the scope of its application every year grows 
wider. These two modes of multiplication are designated respec¬ 
tively by the apt names of homogenesis when the successive 
generations are alike, and heterogeneais when they are diverse. 
But we may look at the matter from another point of 
view—the production of a new individual may result from 
the fusion of the whole or a portion of each of two more 
or less distinct previously existing individuals, from a sexual 
act; or it may originate merely by the separation of a part of 
one individual in such a form as to be capable of independent 
growth. These two modes are called by the delightfully 
expressive names of gomogenesis and agamogenesis respectively. 
Herbert Spencer then lays down the following laws:— 
(1) Homogenesis is always gamogenesis ; (2) Heterogenesis 
is agamogenesis interrupted more or less frequently by gamo¬ 
genesis. These statements we will now examine. 
The kind of liomogenesis with which we are most familiar 
is that met with in the .larger animals, where each genera¬ 
tion consists of males and females; it occurs under three 
forms, as viviparous, oviparous, and the intermediate ovo- 
viviparous genesis. As we descend in the animal kingdom 
we find liomogenesis become rarer and rarer, and at last 
entirely replaced by the other mode. Amongst plants, as will 
be seen hereafter, liomogenesis, strictly speaking, never occurs. 
When lieterogenesis prevails we shall find that after a 
generation of perfect males and females there will be a 
generation of asexual individuals, which produce the next 
generation by a process of budding. This next generation 
may consist of males and females like those with which we 
started, or may be itself also asexual, and produce another 
generation by budding ; but sooner or later the sexual gene¬ 
ration will again occur and complete the cycle. Illustrations 
of both these modes are most readily seen in plants. Take a 
simple uniaxial plant, originating from a seed, and having a 
terminal flower. The stem and leaf appendages of this 
constitute a single individual, which is itself sexless, and pro¬ 
duces buds, each bud being a new individual. From certain 
of these buds, which are male, pollen-grains are formed; 
from certain others, which are female, ovules (or rather 
embryo-sacs) are produced. In certain of the lieterosporous 
higher cryptogams the homologous parts are called micro- 
