To return for a moment to the case of the change from the unicellular to the multicellular, it is obvious that in the new unit, to get the full advantage of the change there must be specialization involving both loss and gain to the individual cell; one loses power of digestion and gains a special sensitiveness to stimulation, another loses locomotion {105} to gain digestion, and so forth in innumerable series as the new unit becomes more complex. Inherent, however, in the new mechanism is the need for co-ordination if the advantages of specialization are to be obtained. The necessity of a nervous system—if progress is to be maintained—early becomes obvious, and it is equally clear that the primary function of the nervous system is to facilitate co-ordination. Thus it would seem that the individual cell incorporated in a larger unit must possess a capacity for specialization, the ability to originate new methods of activity, and a capacity for response—that is, the ability to limit itself to action co-ordinated suitably to the interests of the new unit rather than to those that would have been its own if it had been a free unit in itself. Specialization and co-ordination will be the two necessary conditions for success of the larger unit, and advance in complexity will be possible as long only as these two are unexhausted. Neither, of course, will be of avail without the other. The richest specialization will be of no good if it cannot be controlled to the uses of the whole organism, and the most perfect control of the individual cells will be incapable of ensuring progress if it has no material of original variation to work on.