ns
are like the workmen. Each workman has his own special piece of work to
do, at which he becomes thoroughly expert; and the finished product is
manufactured more rapidly and efficiently by the co-operation of workers
each skilled in one department than it would be if each workman had to
produce the whole. Applied to the organism this principle of the
division of labour means the differentiating out of the separate
functions, their localisation in different parts of the organism, and
their co-ordination to produce a combined result.

This differentiation of functions implies a corresponding
differentiation of organs, but it is functional differentiation which
always takes the lead. "Where division of labour has not been introduced
into the organism there must exist a great simplicity of structure. But
just as uniformity in the functions of the different parts of the body
implies a uniformity in their mode of constitution, so diversity in
function must be accompanied by particularities in structure; and, in
consequence also, the number of dissimilar parts must be augmented and
the complication of the machine increased" (p. 463). Since function
comes before form there is not always a special organ for every
function. "It is a grave error to believe that a particular function can
be performed only by one and the same organ. Nature can arrive at the
desired result by various ways, and when we look down through the animal
kingdom from the highest to the lowest forms we see that the function
does not disappear even when the special instrument provided for the
purpose in the higher types ceases to exist" (p 470).

Nature, holding fast to the law of economy, does not even always create
a new organ for a new function; she may simply adapt an undifferentiated
part to special functions, or she may even convert to other uses an
organ already specialised (p. 464). So, for example, the function of
respiration is in the lowest animals diffused indifferently over the
whole surface of the body, and only as organisation advances is it
localised in special organs, such as gills. Now suppose that Nature
wishes to adapt a fish, which breathes by gills, to life in the air; she
does not create an organ specially for this purpose, but utilises the
moist gill-chamber (_e.g._, in _Anabas scandens_), modifying it in
certain ways so that the fish can take advantage of the oxygen it
contains. But this gill-chamber lung is at best a makeshift, and