this tiny particle preserves all the original cells, or at least enough of them to make a fresh start, and a very sluggish metabolism goes on in each cell. Of course, the advantage of this is that the stored food materials will not be used up as rapidly as they would if metabolism went on at the usual rate, and so there is a better chance that the animal may survive until more water falls or drains into the puddle, or until the particle of dust which the animal has become may be blown by the wind where it will fall into another one. Whenever either of these things happens the protoplasm takes up water again and the former rate of metabolism is resumed. It is only by means of this reduction in rate of metabolism that many kinds of animals are able to persist, for in large parts of the globe there is a period of each year when conditions become so unfavorable that the usual rate of metabolism could not possibly be maintained. Next in order to basic metabolism comes the metabolism of growth, by which we mean the energy necessary for the making of new protoplasm. Not a great deal is known about growth metabolism; in fact, about the only reason for believing that it requires any energy at all is that the metabolism of young animals, whenever it has been studied, has been found to be greater in proportion than that of animals that are fully grown. It is hard to account for this, unless the growth process itself, namely, the making of new protoplasm, requires energy. When we think of the extreme complexity of living protoplasm, we can easily believe that its formation involves the expenditure of energy, perhaps in considerable amounts.