He is as emphatic in his conclusion that some hybrids are perfectly fertile--as fertile as the pure parent-species--as are Kolreuter and Gartner that some degree of sterility between distinct species is a universal law of nature.

He experimented on some of the very same species as did Gartner.

The difference in their results may, I think, be in part accounted for by Herbert's great horticultural skill, and by his having hot-houses at his command.