The City of the Sun Read Online Free PDF Page A

Nathan.
    “Here we have four parasites which form black dendritic webs on the outer skin of their hosts. But all four hosts are small mammals of no economic importance or ecological interest. Rabbits and field mice, as near as damn it.”
    “So?”
    “They didn’t find it in association with the oxen,” I said, patiently. “If they had, they’d have taken a lot more interest in it. The oxen are useful, valuable animals. Their diseases were a matter of considerable import in assessing the potential of a colony here—their presence provided a possible source of meat, transport and farm labor. But who’s interested in rabbits and field mice? The survey team did no more than a routine bioscan on this lot, whereas if they’d found it among the oxen—from which the people here presumably caught it—they’d have looked at it much more closely.”
    “Didn’t they realize it might infect humans?” asked Mariel.
    I shook my head as I studied the cards more carefully, one by one. “They noted that the parasite was probably capable of infecting a range of compatible hosts. They didn’t realize how wide a range. But even if they had, they might not have considered it important. Most people, remember, don’t just sit back and let things grow all over them. They try to do something about it.”
    That, of course, was one of the most worrying things. If the people of the city were all infected by this thing then they obviously hadn’t put up much of a fight. One of the first things the colonists would have done would have been to prepare some biotic defenses against their new environment. Simple medical technology is the first priority of any colony.
    The cards told me that the dendrites ramified internally as well as externally, but only to a limited extent. The bulk of the biomass lay on the surface, with only thin threads—chains of potentially independent cells—linking it to the circulatory system and the nervous system of the host. The parasite was careful not to damage its hosts by too much disruption of the tissues. It didn’t feed on tissues—just leeched what it required from the bloodstream. A very considerate vampire, if appearances were to be believed. The surveyors reported that infected animals were at least as healthy as uninfected ones.
    Then I saw something on one of the cards that made me put the others aside.
    “If anyone wants to bet,” I said, “I’ll lay six to four on this one as the culprit.”
    There were no takers, but they all wanted to know why.
    “It’s the special one,” I told them. “It has a footnote that the survey team didn’t think was especially significant. It says here that this particular species goes in for inductive cellular mimicry. Especially with respect to nervous tissue.”
    “Which means?” Nathan prompted.
    “These communal protozoans are versatile,” I said. “It’s the key to their success. Some protozoan species on Earth are versatile enough to choose whether to be plants or animals—they can grow chloroplasts and dispose of them as circumstances dictate. The whole essence of communal aggregation is that it’s the beginnings of division of labor—some cells specialize in reproduction, others in energy fixation, others in defense. It happens in the colonial algae and in the colonial polyps. The point about communal aggregations, though, is that the cells retain their potential independence—and their potential choices. Organisms—multicellular organisms, that is—go in for a much more precise kind of specialization. Once a cell grows to its destined function it remains specialized. Once a liver cell, always a liver cell—the versatility of each individual cell is lost at an early stage in the development of an embryo when cells become fixed into their permanent function. This process of specialization is involved with a mechanism called induction, which causes different tissues to develop in the right places within the embryo in response to the