a few days everybody will know,
the proof will stare you in the face. And what will happen then?"

"Evidence?" Karl asked. "I can't accept a statement as a fact."

"Would you like to see my mice? Come with me."

David Wong hurried into the small animal room and paused before a stack
of wire cages in which furry creatures darted and squeaked.

"You remember when we were working on Blue Martian, those peculiar
mutants we found in our mice, and how I used six of them in trying to
make antibodies to the virus?"

"I remember," said Karl. "They were spotted with tufts of white hair on
the right forelegs."

David took down a cage, thrust in his hand, and brought out two of the
tiny black mice which crawled over his trembling hand. Their right
forelegs bore tufts of long white hair.

"These," he said, "are the same mice."

[Illustration]

"Their descendants, you mean. Mice don't live that long."

"_These_ mice do. And they'll go on living. For years I've lived in fear
that someone would notice and suspect the truth. Just as for years,
every time someone has laughed and told me I never seemed to age a day,
I've been terrified that he might guess the truth. I'm _not_ aging."

Karl looked dazed. "Well, my boy, you've got a bear by the tail. How did
you find the elixir or whatever it is?"

"You remember the early work with radioactive tracers, a couple of
hundred years ago, that proved that all our body cells are in a
continuous state of flux? There's a dynamic equilibrium between the
disintegration and the resynthesis of the essential factors such as
proteins, fats and amino groups, but the cell directs all the incoming
material into the right chemical structures, under the influence of some
organizing power which resides in the cell.

"Foreign influences like viruses may disrupt this order and cause
cancer. The cells are continually in a state of change, but always
replace their characteristic molecules, and it is only as they grow
older that they gradually become 'worn out.' Then the body grows old,
becomes less resistant to infection, and eventually succumbs to one
disease or another. And you know, of course, that viruses also have this
self-duplicating ability.

"I reasoned that at birth a man had a definite, finite amount of this
essential self-duplicating entity--SDE--in his body cells, a kind of
directing factor which reproduces itself, but more slowly than do the
body cells. In that case, with the normal