POPULAR SCIENCE REVIEW. 
34 
6. Male individuals ; 
7. Female individuals. 
These individuals are not distinguished only by the functions 
they fulfil ; they are also distinguished by their external form, 
so that a peculiar kind of individual corresponds to each special 
function. Each of them, so to speak, acquires the form of 
its employment, at the same time rising, or becoming retrograde 
in the scale of organization, so that here, as in human societies, 
the division of labour superinduces differences of condition. The 
species thus become polymorphic. 
Of the seven kinds of individuals composing a colony of 
ITydractinia}, the nutritive individuals alone seem capable of 
being self-sufficient. The others are destitute of mouth and 
of tentacles ; the sexual individuals are reduced to simple sacs ; 
the defensive individuals seem to be only horny spines, 
between which the poljqjes ean retract themselves. In pre- 
sence of these facts, it would seem to be an exaggeration to 
attribute the quality of individuals to these different parts ; 
we have here, it might be said, simple organs. But organs of 
what? They are just as independent of each other, just as 
independent of the nutritive individuals as the latter can be of 
one another. Hence they are not organs of those polypes. 
Are we to see in them organs of the colony ? This is at once 
to recognize that the colony has an individual character, and 
consequently to assume the transformation that we seek to 
demonstrate. But how has a colony been able to acquire such 
organs ? whence can they have arisen if not from a transforma- 
tion of the individuals composing it ? 
However, we have no occasion for hypothesis in order to 
demonstrate that these colonial organs are the equivalents of true 
individuals. The buds which give origin to the different kinds 
of individuals in a colony of Hydractiniae all originate in the 
same way, and they are for a long time so similar that there is 
nothing to enable one to distinguish them. This furnishes a 
first presumption in favour of their equivalence ; but in the 
allied type Podocoryne we see the bumble sac which represents 
the sexual individual replaced by a most active and elegant 
creature, much higher in organization than the Hydra itself, 
namely, by a Medusa, which detaches itself on its arrival at 
maturity, and swims about actively in the water, all the 
transparency of which it possesses. 
These Medusae constitute the most general form of the 
sexual individuals in the group of Hydroid Polypes, but they 
are themselves very multiform. From species to species their 
form is modified ; we see them stop at every degree of develop- 
ment, sometimes becoming reduced to the condition of a mere 
bud, sometimes, although completely formed, abstaining from 
