As is the case with animal and human species, very often specialized groups start small and as they prosper, they get bigger. Irish boxers from their home island of Ireland dominated the flyweight and bantamweight divisions for decades in the mid-20th century.

But Irish-American boxers tended to do best in the heavyweight division as early as the time of John S. Sullivan more than a century ago. When the Irish migrated to America, over the generations, they became more prosperous and successful and consequently, a lot bigger.

The same evolutionary principle -- that success begets bigger size, can be seen in a whole host of land, sea and air weapons systems. Today's nuclear powered U.S. super-aircraft carriers dwarf the highly successful -- and in their own day, breathtakingly large -- Essex-class carriers that ruled the Pacific Ocean during World War II.

Modern Main Battle Tanks like the U.S. Abrams M12A and the Russian T-90S dwarf most of their World War II evolutionary grandfathers. The famous World War II single engine, propeller-driven fighters like the British Supermarine Spitfire, the U.S. North American P-51 Mustang, the Yakovlev Yak-3 fighter and the German Messerschmitt Me-109 or Focke Wulf-190 would be dwarfed in size, as well as outclassed in speed and firepower, by a modern F-22 raptor, F-35 Lightning II or Russian Sukhoi Su-30 or MiG-29.

However, giantism in weapons systems, as in animal and human evolution, has its limits. Eventually a point is reached where a weapon can be too big, too slow and therefore too vulnerable to survive the many attacks it will face from far smaller but deadly predators that will outnumber it and assail its defenses on every side.

U.S. aircraft carriers were designed to carry more and better aircraft than any other carrier force afloat, and for 65 years, as they have gotten bigger and bigger, they have become and more dominant until today, no other nation in the world has plans to build enough surface aircraft carriers to even dream of challenging the dominant U.S. ones.

But instead, the gigantic U.S. ships must face the looming 21st century threat to their survival from lots of cheap, easily built diesel submarines. Russia and China have bet big on this option, as we have repeatedly noted in these columns, and other countries like Iran and Indonesia are trying to follow in their paths, too.

Similarly, the main challenge in the first wars of the 21st century to the mighty General Dynamics M1 Abrams MBT comes not from its supposed arch rival, the Russian T-90, nor even from rocket-powered grenade launchers. And they were already a formidable presence in the Nazi Wehrmacht of 1944-45 as the familiar Panzerfaust.

Instead, the main threat to the U.S. land leviathans in urban environments has come from the simple shaped-charge improvised explosive device that has been used so effectively by Sunni Muslim insurgents in central Iraq.

IEDs have certainly not made the main battle tank obsolete. But they have revived the importance -- completely understood by the great German panzer generals, Heinz Guderian, Erich Von Manstein and Erwin Rommel, during World War II, and by the best of their allied opponents like George S. Patton, Omar N. Bradley and British Field Marshal Bernard Montgomery -- of making sure tanks only launch attacks as part of carefully integrated operations combined with interactive infantry and artillery forces as well.

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Next: Weapons evolution trends in the 21st century