In the beginning, there existed no other animal organisms than these aggregations of similar unicellular beings, all of which reproduced themselves.  Later on, division of labor made its appearance among the individuals of the animal colony, and it increased their dependence upon one another, so that their individuality was to a great extent lost, and they were no longer able to live independently of one another.

By the development of this process, multicellular metazoa arose from the colonies of similar protozoa, and at length culminated in the higher animals and man.

If we examine the human body, its origin and end, in the light of these facts, we shall see that a comparison between the simple immortal protozoa and man leads us to the result that man himself, or at least a part of him and that the most important, is immortal.

When we turn to the starting point of human development, we find an egg cell and a spermatozoon, which unite and whose nuclei intermingle.  Thus a new cell is produced.  This process is similar to the conjugation of two unicellular beings, such as two acinetiform infusoria, one of which, the female ([Symbol:  Female]), is larger than the other, the male ([Symbol:  Male]).  This difference of size in the conjugating cell is, however, without importance.

From this double cell produced by conjugation many generations of cells arise by continual cell division in divergent series.  Among the infusoria these are all immortal, but many of them are destroyed, and only a few persist till conjugation again takes place.  The same is the case with man.  Numerous series of cell families arise, which are all immortal:  of these but few—­strictly speaking, only one—­live till the next period of conjugation and then give the impulse which results in the formation of a new diverging series of cells.  The difference between man and the infusorian is only that in the former the cells which originate from the double cell (the fertilized ovum) remain together and become differentiated one from another, while in the latter the cells are usually scattered but remain alike in appearance, etc.

The seeds of death do not lie, as Weismann appears to assume, in the differentiation of the cells of the higher animals.  On the contrary, all the cell series, not only those of the reproductive cells, are immortal.  As a matter of fact all must die; not because they themselves contain the germs of death and have contained them from the beginning, but because the structure which is built up by them collectively finally brings about the death of all.  The living plasm in every cell is itself immortal.  It is the higher life of the collective organism which continually condemns countless cells to death.  They die, not because they cannot continue to exist as such but because conditions necessary for their preservation are no longer present.

Thus, while the cells are themselves immortal, the whole organism which they build up is mortal.  The complex inter-dependence between the single cells, which, since they have adapted themselves to division of labor, has become necessary, carries with it, from the beginning, the seeds of death.  The mutual dependence ceases to work, and the various cells are killed.