"And," asked Captain Burke, "you say these waves are deadly to humans? But how, then, have our colonists managed to win and maintain a foothold--"
"I should have said," admitted Grossman, "the waves are deadly to _unshielded_ humans. Lead sheathing protects the wearer from harm; consequently men in bulgers are quite safe. And one of the first acts of the Solar Space Patrolmen, upon reaching Titan, was to project a series of leaden highways or avenues between the cities of the satellite. Upon these, and _only_ upon these, may Earthmen travel unprotected by bulgers. To stray from one of these roadbeds means exposure to the T-radiation. And that, in turn, means death!"
Rockingham Roswell shuddered delicately. "Beastly!" he murmured. "Deuced unpleasant sort of place, what? But, I say ... how about the natives? How did they manage to survive before our countrymen built those jolly old lead roadways?"
Grossman pursed his lips impatiently at the affected young scholar.
"They, Doctor Roswell," he said scornfully, "are immune to the T-radiation. Certainly you are acquainted with the principles of selective breeding?"
"Selective--oh, yes! Survival of the fittest ... all that fiddle-di-diddle? You mean the present Titanians _are_ the present Titanians simply because they adapted their physiques to the surroundings, eh? Why, rather! That's clear enough. Still, if they can stand the radiation, I don't see why other humans--"
"Other _humans_!" Grossman laughed curtly. "My dear Doctor, it is obvious you have never seen a Titanian. Human, indeed! Why, it is the dissimilarity between the Titanians and ourselves which led me to name racial divergence as among the hazards of life on Titan.
"The creatures who rule Titan look less like humans than like those monsters deranged and alcoholic patients see in their dreams. For some reason--possibly because of this mysterious T-radiation--the denizens of the world have never bred true. Consequently, there is no way of foretelling what the child of any two parents may resemble ... though one almost certain guess is that it will resemble neither parent.
"Bilateral symmetry is about the only constant human attribute to be found amongst the Titanians. That and a more or less rudimentary intelligence ... an instinct which is more akin to animal cunning than to intellect.