They came from an old ingrowing world which had never reached the stars, where there were only men and their works, no blade of grass or micro-organism or sparrow which did not directly serve men. In their time, hereditary traits which had meant untimely and certain death in earlier times had persisted and multiplied. Immunities and instincts which had fitted men to live with tigers and streptococci, and seek their food in the wilderness, had atrophied.
The twentieth century was a dangerous environment for these people, more so perhaps than the Eocene would have been for homo sapiens. In preparation for their venture, it had been necessary for them to undergo a drastic and painful series of tests, inoculations, conditionings and plastic surgery.
Unfortunately, it had not occurred to them that their time machine might need similar protection. The equipment was basically electronic, and the power leads were encased in a new insulation, a synthetic protein which in very thin films afforded a near perfect dielectric. It was also, as it happened, an almost perfect culture medium for certain bacilli, non-existent in the sterile future, but healthy and thriving and full of appetite in the twentieth century.
When the gun runners prepared to return to their own time with their cargo of contraband there were small flashes of fire, and smoke curled briefly from various parts of the equipment. Their temporal environment remained unchanged.
The gun runners were not technicians, they were specialists in other fields. They pulled and prodded uncertainly here and there, pushed the buttons again.
Nothing happened.
The senior gun runner, a man who wore in this century the appearance of a quiet, gray-haired professional man, and who wore in any century the habit of command, came to a decision. He spoke in their own language, a language time had pruned to telegraphic brevity:
"If tamper, make worse. Electronics technicians this era. Use."
The second man raised an eyebrow. "Knowledge adequate? Time travel not simple."
The older man shrugged. "Theory not simple, machine simple. Savages clever fingers. Adequate stimulus, can solve."