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 when she comes to the more definitely terrestrial Amphibia Nature gives up the attempt to use the gill-chamber as a lung, and creates a new organ, the true vertebrate lung, specially adapted for breathing air (p. 475). But whatever means Nature adopts, her aim is always the same--to specialise, to differentiate, to produce diversity from uniformity.