Xenology

For "The Master Key," see here.

Discussing the Cainites:

"'They're mammals, more or less,' Per said..." (p. 123)

They cannot be mammals per se but they are products of parallel evolution.

Per Stenvik continues:

"'...though with any number of differences in anatomy and chemistry, as you'd expect.'" (ibid.)

That there are many differences is a predictable generalization but can he be more specific? Yes, he is in the very next sentence:

"'They don't sweat, however. There's a complicated system of exo- and endothermic reactions in the blood to regulate temperature.'" (ibid.)

In response to that, van Rijn makes an astounding generalization:

"'Sweating is not so common on cold terrestroids...'" (ibid.)

OK. Technic civilization science includes xenology. Xenologists are familiar with large numbers of:

terrestroid planets;

cold terrestroid planets;

inhabited cold terrestroid planets;

many inhabitants of such planets who do not sweat;

a smaller number who do sweat.

On the basis of all that knowledge, van Rijn is able to make the generalization that sweating is not common on cold terrestroids! 

Are there that many inhabited planets? Will any interstellar civilization ever be able to study them in that amount of detail? Some later works by Poul Anderson are based on different premises.