Polaris Venture Partners has quietly closed its sixth fund with $375 million in capital commitments. The actual event happened months ago, but Polaris doesn’t usually talk about fund closes until it’s ready to begin investing the new money (which it now is).

I spoke briefly last night with Polaris co-founder Terry McGuire, while he was shopping at a supermarket in his hometown of Buffalo. He says the Waltham, Mass.-based firm’s tri-furcated investment strategy will remain consistent, with one-third going to tech, one-third to life sciences and one-third to growth equity. He also says that the firm recently made its final new investment out of Polaris Venture Partners V, a $1 billion fund raised in early 2006.

As you probably noticed, the new fund is much, much smaller than the last one. McGuire says that $1 billion proved too large for Polaris, as evidenced by how long it took to invest. He’s right, of course, but the new size also reflects a very difficult fundraising environment. Polaris originally told people that it was seeking $500 million, but later lowered the target to $400 million. You easily could argue that this fund, more than its outsized predecessor, will determine the firm’s ultimate future…