This greater variability in mongrels than in hybrids does not seem at all surprising.
For the parents of mongrels are varieties, and mostly domestic varieties (very few experiments having been tried on natural varieties), and this implies that there has been recent variability; which would often continue and would augment that arising from the act of crossing.
The slight variability of hybrids in the first generation, in contrast with that in the succeeding generations, is a curious fact and deserves attention.