288 
PHYSIOLOGY. 
BOOK II. 
are the most remarkable, there is much room for doubt 
whether the supposed species upon which the argument is 
founded are any thing -more than wild varieties of each other. 
The African Gladioli are known to intermix freely ; but 
Herbert, in his account of them, in the Horticultural Trans- 
actions, vol. iv. p. 16., admits that he cannot speak to the 
power of their mules to perpetuate themselves by seed. No 
botanist can fix positive characters to a large part of the 
reputed species of Pelargonium, or to the South American 
Amaryllises ; many of the supposed species of Crinum seem 
to have no better claim to be so considered than the varieties 
that might be picked from a bed of tulips ; and, lastly, the 
Tritica caerulescens, polonicum, and tomentosum, upon which 
Bellardi's experiments were founded, are plants with the 
history of which no man is acquainted, and which, in all 
probability, derive their origin from the Triticum aestivum, 
or common wheat. 
All, I think, that can be conceded upon this subject is, that 
more hybrid plants are fertile to the third or fourth generation 
than Kolreuter supposed, and that the degree of their sterility 
will depend very much upon the degree of natural relationship 
which their parents may have possessed. That they will all, in 
time, revert to one or other of their parents, or become abso- 
lutely barren, there can be no doubt whatever. 
Although this power of creating mule plants that are fertile 
for two or three generations incontestably exists, yet in wild 
nature hybrid varieties are far from common ; or, at least, there 
are few well-attested instances of their occurrence. Amonor 
the most remarkable cases, are the Cistus Ledon, constantly 
produced between C. monspessulanus and laurifolius; and 
Cistus longifolius, between C. monspessulanus and populi- 
folius, in the wood of Fontfroide, near Narbonne, mentioned 
by Bentham. The same acute botanist ascertained that Saxi- 
fraga luteopurpurea of Lapeyrouse, and S. ambigua of De 
Candolle, are only wild accidental hybrids between S. are- 
tioides and calyciflora : they are only found where the two 
parents grow together ; but there they form a suite of inter- 
mediate states between the two. Gentians, having a similar 
origin, have also been remarked upon the mountains of 
