130 
PRINCIPLES OF BIOLOGY. 
great instability, i.e., in this case, great capacity for growth, 
in the product. Too great a difference is as ineffective as 
too little. 
We know from the instance of Begonia that a single cell 
can contain within itself all the physiological units necessary 
for the reproduction of a species. Assuming then that the 
fertilised germ contains all the required units—derived 
jointly from both parents, and in a state suited for further 
growth—let us test the hypothesis by comparing it with 
established facts. 
In the first place, Heredity and Variation become mere 
matters of course. The offspring cannot but resemble at the 
same time that it differs from its parents. Then the superi¬ 
ority of cross fertilisation to self-fertilisation is manifest in 
the greater unlikeness between the combining units which it 
ensures, and the consequent greater vitality of the offspring. 
Upon these, which are so fully treated in the original, it is 
not necessary to enlarge. Though self-fertilisation is not 
impossible, yet it is probable that it could not go on for ever; 
the species would in most cases die out. A fortiori, then 
agamogenesis could not go on for ever ; yet as we have seen 
before, so far as our present knowledge extends, there seem 
to be cases in which no gamogenesis ever occurs. The cases 
of plants propagated, as it would appear, indefinitely by buds 
or cuttings or offsets, e.<j., the prolific banana, in which it is 
said no seeds have ever been produced within historic times, 
as well as those extremely numerous cases of fungi which were 
mentioned in a previous chapter, all show that the theory so 
far presented, though true, is not the whole truth. 
We find agamogenesis prevailing the more the lower the 
type of the organism, the less differentiated its parts, the 
simpler and more uniform the conditions under which it lives; 
and this view is confirmed by observing that the only large 
class of plants in which no gamogenesis is known in any of 
its members is the lowest of all, the Bacteria. Hence it is 
obvious that the need for gamogenesis increases in proportion 
to the complexity of the forces which act upon the organism. 
Now, what causes a species to continue to live ? Its fitness 
for its environment. If then this fitness can be easily main¬ 
tained on the average, if the species exhibits no great 
tendency to vary in such a way as to unfit itself for its con 
ditions of life, if in fact the forces which act upon it are not 
relatively complex, then it can maintain its position for long 
periods even by agamogenesis ; but if the conditions are 
complex the individuals must be severally acted upon 
in different ways by their special environments, they must 
