here, where the rhythms are very slow, they are individually less regular, and are united into a less regular compound rhythm--are less integrated. Perhaps the clearest general idea of the co-ordination of functions that accompanies their specialization, is obtained by observing the slowness with which a little-differentiated animal responds to a stimulus applied to one of its parts, and the rapidity with which such a local stimulus is responded to by a more-differentiated animal. A sea-anemone and a fly will serve for the comparison. A tentacle of a sea-anemone, when touched, slowly contracts; and if the touch has been rude, the contraction presently extends to the other tentacles and eventually to the entire body: the stimulus to movement is gradually diffused throughout the organism. But if you touch a fly, or rather if you come near enough to threaten a touch, the entire apparatus of flight is instantly brought into combined action. Whence arises this contrast? The one creature has but faintly specialized contractile organs, and fibres for conveying impressions. The other has definite muscles and nerves and a co-ordinating centre. The parts of the little-differentiated sea-anemone have their functions so feebly co-ordinated, that one may be strongly affected for some time before any effect is felt by another at a distance from it; but in the much-differentiated fly, various remote parts instantly have changes propagated to them from the affected part, and by their united actions thus set up, the whole organism adjusts itself so as to avoid the danger. These few added illustrations will make the nature of this general relation sufficiently clear. Let us now pass to the interpretation of it.