How far the combination of these individual differences of chemical, and at the same time vital, organization is able to quicken, to strengthen the metabolism, to bring about 'physiological regeneration,' or whatever we may choose to call it, we do not yet understand. It has been said that in plastogamy an exchange of 'substances' takes place; that each gives to the other the substances which it possesses and the other lacks, and that this causes an increase of vital energy. But it is unlikely that we have here to do merely with chemical substances, although these, of course, as the material basis of all vital processes, are indispensable; it seems to me more probable that the vital units (biophors) themselves in their specific individuality must play the chief part. But even this is saying very little, for we have not yet reached an understanding of these processes, and if we were not forced by the fact of plastogamy to the conclusion that this union must have some use, no one would have been likely to postulate it as useful, still less as necessary. It has, of course, been frequently suggested that multiplication by fission, if long-continued, results in 'exhaustion,' and that this is corrected by amphimixis, but who can tell why this 'exhaustion' might not be remedied, and even more effectually remedied, by a fresh supply of fuel, that is, of food? One might have thought that the vital processes would be thus more readily recuperated than by the co-operative combination of two already 'exhausted' cells. Two exhausted horses may perhaps be able to pull the load that one of them was no longer equal to, but in the case we are considering it is the combined burdens of two units that have to be borne, although each was no longer equal to its own share! That is more than we can understand.