What does the bold "it" in the italicized line refer to, the bacterium or a desirable chemical?
Anyway the sentence containing it evades me. What does the sentence mean?
A bacterium is so small that its sensors alone can give it no indication of the direction that a good or bad chemical is coming from. To overcome this problem, the bacterium uses time to help it deal with space. The bacterium is not interested in how much of a chemical is present at any given moment, but rather in whether that concentration is increasing or decreasing. After all, if the bacterium swam in a straight line simply because the concentration of a desirable chemical was high, it might travel away from chemical nirvana, not toward it, depending on the direction it’s pointing. The bacterium will swim in a straight line as long as the chemicals it senses seem better now than those it sensed a moment ago. The bacterium solves this problem in an ingenious manner: as it senses its world, one mechanism registers what conditions are like right now, and another records how things were a few moments ago. If not, it’s preferable to change course.