314 
THE PIIINCIPLES OF BIOLOGY. 
as two separate ones? and, in the latter case, when exactly 
does the separate individuality of the cut-off branch begin ? 
In the case of plants which send out runners, which make 
roots and grow into separate plants, we have the same 
questions to answer. 
In the animal kingdom we find still greater difficulties. 
All the numerous cases of compound Hydrozoa present us 
with almost an exact analogy to the tree just spoken of. We 
find young perfect animals budding out from the parent 
animal, but remaining in permanent connection with it. Are 
we then to consider each polype as an invividual, or are we 
to give that signification only to the whole colony ? The 
difficulty increases when we find that in many cases the 
individuality of each member becomes partially merged in the 
individuality of the colony. We find, in fact, an association 
on the principle of division of labour, some members attending 
to the locomotion, some to the procuring of food, some to 
digestion, some to reproduction, &c. Probably in these cases 
we are witnessing one of the steps by which Nature proceeds 
in the creation of new organisms of a higher development 
. and differentiation. We may suppose this merging of the 
single individuality into the aggregate individuality, and the 
application of the division of labour, or rather of function, to 
go on increasing, and thus we have the material out of 
which to construct a being of almost any amount of organisa¬ 
tion and differentiation. Have we not in the articulate 
animals a structure which might have been brought about by 
a similar process ? 
As a definition of an individual it has been proposed to 
give that name to the whole product of one single fertilised 
germ ; but there are many difficulties and much inconvenience 
attendant on that course. It is a clear and satisfactorv 
definition in all cases where one fertilised germ produces only 
one separate perfect animal. But there are cases such as the 
Medusae, and the Aphides, where one germ gives rise to a 
multitude of perfect animals. Are we then to consider all 
these animals as parts of one individual ? The proposition 
to call an individual each perfect animal which has the power 
to reproduce its kind is met by similar difficulties, in the case 
of bees, ants, &c., where we find perfect and separate animals 
unable to do so. If they are not individuals, what are they? 
and what are we to do in the case of insects where the animal 
attains the power of reproduction only in the mature stage ? 
Thus we are forced to the conclusion that no perfect definition 
is possible, and that the best course is to make a compromise. 
We must consider that with the idea of an individual is 
