PHYSIOLOGY. 397 
IS) 293. 
Koelreuter, however, could only obtain. hybrids 
by intermixing similar plants. Dissimilar plants 
never produced them, even though, according to 
our systems, they belonged to one genus. It ap- 
pears that nature thus avoids unnatural mixtures. 
The instance of mules not generating, as it was 
once believed at least, induced many philosophers 
to make it an axiom, that hybrids are barren. But 
we now know a good many instances in Zoology of 
hybrids being very productive, and even the in- 
stance of mules does not prove any thing, as in 
warm climates they are sometimes prolific. 
Koelreuter likewise found hybrids of various spe- 
cies of tobacco and some more plants to be sterile, 
the pistil in them being very perfect, but the sta- 
mens not completely formed. But there are now 
several instances of hybrid plants which retain their 
original form, and propagate themselves. I shall 
only mention a few with their parents: 
Sorbus hybrida. ‘TYhe mother was Sorbus aucuparia ; 
the father, Crataesus ria. 
Pyrus /ybrida. ‘The mother was Pvyrus ardutifolia; 
the father, Sorbus aucuparia. 
Rhamnus hybridus. ‘Vhe mother was Rhamnus a/. 
pinus ; the father, Rhamnus A/aternus. 
What mixtures do not the species of Pelargonium 
produce in our gardens? All plants of the 21st, 
22d, and 23d classes of Linné mostly generate pre- 
lific hybrids. Linné wrote a particular treatise on. 
, Pypeids, in which he attempted to explain the origin 
x 4 ot 
