It all comes down to the songs.

Lots of 'em.

Chicago, the group (not the city), has enjoyed more than four decades of hit songs, shared in recordings, concert tours and on the radio. These yielded a fan base and tons of memories — galvanizing the group's grip as a prolific musical force.

As founding member and singer Robert Lamm puts it: "It's not the number of years that matters; it's the body of work that continues to apparently gain new fans. The music is nostalgic for some, but our 20-something-year-old fans say it's the songs, the skill of singing/playing the music and the energetic live shows."

The eight-member band with Windy City ties is making yet another Island stop, performing at Blaisdell Arena on Saturday, after a Japan tour the past several days.

Lamm, 56, has been at the heart of Chicago's birth and has seen it emerge as a mainstream band. He fielded some e-mailed questions for this story.

He said Chicago's treasured, classic songs — still staples on some radio playlists — have kept the group in demand as an A-list act, not a nostalgia commodity.

"We figured out that there were about 30 to 35 Top 20 hits and 30 to 35 other Top 40 (hits)," he said. "So our entire show is about songs that people love hearing, from the first album through 'Chicago XXX.'

(Chicago's albums, from vinyl to the CD era, have generally carried the number as part of the title, with a few exceptions).

"Occasionally, we challenge the audience's ears and throw in something fresh, but the reason we appeal so broadly is we have written timeless music, without realizing we were doing so," Lamm said.

The group was founded in 1967 as Chicago Transit Authority, originally with an experimental, more rock-oriented sound than today's softer hybrid developed in the 1970s and '80s. Chicago is known for rockaballads — with elements of jazz, Latin, pop and soul. After the group's debut album, Transit Authority was dropped from its name when threatened by a suit from the city's real transit authorities.

Chicago's stable membership, with few changes, has contributed to its durability. The group is second only to The Beach Boys in the number of singles and albums charted in U.S. pop music history. Where The Beach Boys evolved into only one original member in concert performances today, Chicago boasts four founding members (Lamm, James Pankow, Lee Loughnane and Walter Parazaider) with only one founding member (Peter Cetera) exiting to successfully launch a solo career.

"And FYI, we have not had a change in the lineup since 1994 — 14 years — which is longer than most bands stay together," Lamm said.

"The most significant change was 24 years ago, when Jason Scheff joined. So I'm pretty sure the lineup had been fairly constant enough for the fans to back."

From the get-go, Chicago's albums have always been about hits. The debut album, a double-disc entity, was a rarity for a first-timer, but it produced hits such as "Questions 67 and 68," "Beginnings" and "Does Anybody Really Know What Time It Is." The group's second album, "Chicago II," proved to be a groundbreaking star-maker, boasting fare — "Color My World" and "25 or 6 to 4" — that became the kinds of classics Lamm says helped lay the foundation for Chicago's longevity.

The group reached the coveted No. 1 slot in the Top 100 five times in its career, for tunes that include "If You Leave Me Now" in July 1976, "Hard to Say I'm Sorry" in May 1982, and "Look Away" in September 1988.

The pair of Island shows (Saturday at Blaisdell and tonight on Maui) are among the 100 concerts Chicago will be doing this year, domestically and abroad. It's a format that has worked for the group for years.

A lot's in store for Chicago (and fans) the rest of the year.

In May, Chicago will tape A&E's "Private Sessions," a series that looks at top musicians, with conversations and performances.

In June, Chicago will launch its annual tour of outdoor venues, partnering with the Doobie Brothers. The gig echoes an association with Earth, Wind & Fire in 2004 and 2005, creating one of the most inspired pairings in pop music and resulting in a DVD, "Chicago and Earth, Wind & Fire: Live at the Greek Theatre."

And Rhino Records — which last October released "The Best of Chicago: 40th Anniversary Edition" — will search the vaults again for a pair of new releases of old material: "Stone of Sisyphus," an unreleased underground fave for 15 years with bonus tracks and updated liner notations, and a box edition of 7-inch Chicago singles targeting the collectibles market.

Chicago has logged record sales of more than 120 million, responsible for an uncanny 50 hits in the U.S. That kind of chart activity is eclipsed only by the Rolling Stones.

Of course, honors have been abundant.

Chicago has been a Grammy winner and multirecipient of the American Music Award; it has a Star on the Hollywood Walk of Fame and a street named after the group in Chicago. In February 2007, Chicago joined the elite short list of artists honored by the National Association of Recording Merchandisers with the Chairman's Award for Sustained Creative Achievement.

Sustain is what the group has done extremely well.

"This is a life spent making music and it is honorable to continue, perhaps inspire," said Lamm. "Forty years ... 50 years ... (this is) our greatest pleasure."

The group evolved when fans were open to a new breed of sound, when acts were finding their niche in pop music. The Beach Boys did surf music, America focused on lush harmonies, Chicago's jazz riffs and bold brass had a particular potency.

"Re the biz, yes, it's changed irrevocably," Lamm said. "But it's never been easy to break through as a band or an artist, so that part has not changed. Young artists must stay focused, play a lot, and stay true to who they are, not try to be like what they think will 'sell.' "

While members no longer live in Chicago exclusively, the group adores the Windy City still. "We love returning to play, visit friends, enjoy the cuisine and the Cubs," Lamm said.

With a repertoire that's familiar, lasting and still very much alive with a growing fan base, why not venture into the realm of a Broadway musical that uses the group's idiosyncratic and iconic songs, the way ABBA's melodies envelop the plot line of "Mamma Mia" and Billy Joel's classics defined the dance spectacle "Movin' Out"?

"There is a Chicago-related project already under way," said Lamm, minimizing details.

So don't be surprised if "Saturday in the Park," "No Tell Lover," "Hard Habit to Break," "You're the Inspiration," "Wishing You Were Here," "Color My World" and "Baby, What a Big Surprise" all wind up in a Broadway spectacle.