42 
PRINCIPLES OF BIOLOGY. 
spores and macrospores respectively. Each of these sexual 
individuals undergoes further growth (reduced to a very 
small amount in the Phanerogams), and then, by the union 
of their products, i.e ., the sexual organs, a seed is produced. 
The cycle, if I understand it rightly, is thus represented by 
A, B ; A, B ; etc. In multiaxial plants the only difference 
is that fresh individuals may be produced by budding for 
many generations before the sexual generation recurs. In 
Ferns and Mosses the same arrangement is seen except that 
a spore in these cases is usually capable of giving rise to 
both male and female organs, though even here we meet 
with a decided tendency to unisexuality. Analogous cases 
to all these are met with in the lower groups of the animal 
kingdom. The agamogenesis, i.e., the production of buds, 
may take place in different way which are called metagenesis 
(both external and internal), parthenogenesis, and pseudo¬ 
parthenogenesis, but space forbids me to enter into these. 
It is obvious that the classification given by Herbert 
Spencer does not exhaust the possible modes of occurrence 
of homo- and hetero-genesis. It is conceivable (1) that 
an organism should multiply by continued agamogenesis, 
which might be either continual liomogenesis or that com¬ 
bined with heterogenesis; and (2j, that when gamogenesis 
recurs it might recur at more than one point in the cycle and 
under more than one form. I am not aware that any 
instance is known of the latter mode, but it is at least 
possible. There are, however, numerous instances which 
seem to fall under the first head, and in which proof of the 
recurrence of gamogenesis is wanting, although in many 
cases it is probable. In the realm of Fungi, e.g., the whole 
series of the Bacteria and Yeast-fungi, the greater part of 
the so-called “Fungi Imperfecti”—the Hypliornycetes and 
Coniomycetes—the Basidiomycetes to which the larger Fungi, 
the mushroom, &c., belong, and scattered examples in other 
groups, would fall, so far as is at present known, under this 
head. In most of these cases we can only account for the 
seeming absence of gamogenesis by supposing either that it 
occurs in some form which has hitherto eluded research, or 
that these are merely parts of the life cycle of some other 
organisms with which their connection is as yet unsuspected 
or unproved, and in which the gamogenesis will be found. 
The subject will be better discussed in connection with a 
future chapter. 
The essential act of gamogenesis is the “ union of two 
centres or cells produced by different parent organisms.” We 
find all possible stages of this union, from the fusion of two 
