Let’s call it a form of political natural selection.

The presidential primary campaign slogs on from state to state and as each state’s votes are counted, the candidates at or near the pack of the pack, bottom of the heap, end of the line — whatever — find themselves with little to zero support.

Why keep fighting?

Two more Republicans threw in the towel today: Chris Christie and Carly Fiorina called it a campaign. They’re suspended, which is a nicer way of saying they’re kaput.

Now it’s up to the GOP grownups to march on.

Except that the remaining candidates isn’t composed entirely of grownups. Donald J. Trump is the frontrunner. Enough said there.

The Democrats started this race with five hopefuls. Three of them dropped out. Now it’s just Hillary vs. Bernie.

But the Republican field still comprises seven candidates. For the life of me I’m not sure why Jim Gilmore is still in the hunt. Ben Carson also must be assessing his chances of ever attaining the White House. Jeb Bush? I had high hopes for him; silly me. Before he bid adieu to this campaign, Christie did a masterful job of peeling the bark off of Marco Rubio at the most recent GOP debate.

Let’s see, who’s left . . . after Trump?

Don’t get me started on the Cruz Missile, Sen. Cruz of Texas. My disdain for him rivals how I feel toward Trump.

My main man is still John Kasich, the serious-minded Ohio governor who’s now trying to call himself a “staunch conservative,” when in reality he’s demonstrated a nice bipartisan touch that today’s hard-core right-wingers deem to be soft, squishy, RINOish.

There will be more Republican dropouts in the next few days. Then we’ll be down to the real serious contenders.

Just as natural selection works wonders in the wild, it does have its way of creating a race where only the fittest can compete.