rapid, the fatal condition is reached soon; if it is slow, the fatal condition is postponed. And cytomorphosis in various species and kinds of animals must proceed at different rates and at different speeds at different ages. Birds grow up rapidly during their period of development; the cell change occurs at a high speed, far higher than that which occurs in man, probably, during his period of development. But after the bird has acquired its mature development, it goes on almost upon a level for a long time; the bird which becomes mature in a single year may live for a hundred or even more. There can be during these hundred years but a very slow rate of change. But in a mammal, a dog or a cat, creatures of about the same bulk as some large birds, we find that the early development is at a slower rate. The animals take a much longer period to pass through their infancy and reach their maturity, but after they have reached their maturity they do not sustain themselves so long. Their later cytomorphosis occurs at a higher speed than the bird's. This is a field of study which we can only recognize the existence of at present, and which needs to be explored before, to any general, or even to a special scientific, audience, any promising hypotheses can be presented. Definite conclusions are of course still more remote.
Next as regards death. The body begins its development from a single cell, the number of cells rapidly increase, and they go on and on increasing through many years. Their whole succession we may appropriately call a cycle. Each of our bodies represents a cell cycle. When we die, the cycle of cells gives out, and, as I have explained to you in a previous lecture, the death which occurs at the end of the natural period of life is the death which comes from the breaking down of some essential thing—some essential group of members of this cell cycle; and then the cycle is broken up. But the death is the result of changes which have been going on through the successive generations of cells making up this cycle. There are unicellular organisms; these also die; many of them, so far as we can now determine, never have any natural death, but there are probably others in which natural death may occur. It is evident that the death of a unicellular organism is comparable to the death of one cell in our own bodies. It is not properly comparable to the death of the whole body, to the ending-up of the cell cycle. Is there anything like a cell cycle among the lower organisms? among the protozoa, as the lowest animals are called? It has been maintained by a French investigator, by the name of Maupas, that such a cycle does exist, that even in these low organisms there is a cell which begins the development, and that gradually the loss in the power of cell multiplication goes on until the cycle gives out and has to be renewed by a rejuvenescent process, and this rejuvenating process he thinks he has found in the so-called conjugating act of these animals, in which there occurs a curious migration of the nucleus of
