348 
PHYSIOLOGY. 
BOOK II. 
sexes in plants is, the property the latter consequently possess 
of producing mules. It is well known, that, in the animal 
kingdom, if the male and female of two distinct species of the 
same genus breed together, the result is an offspring inter- 
mediate in character between its parents, but uniformly in- 
capable of procreation, unless with one of its parents ; while 
the progeny of varieties of the same species, however dis- 
similar in habit, feature, or general characters, is in all cases 
as fertile as the parents themselves. A law very similar to 
this exists in the vegetable kingdom. 
Two distinct species of the same genus will often together 
produce an offspring intermediate in character between them- 
selves, and capable of performing all its vital functions as 
perfectly as either parent, with the exception of its being 
unequal to perpetuating itself permanently by seed ; should 
it not be absolutely sterile, it will become so after a few 
generations. It may, however, be rendered fertile by the 
application of the pollen of either of its parents ; in which case 
its offspring assumes the character of the parent by which the 
pollen was supplied. This power of hybridising appears to 
be far more common in plants than in animals ; for, while 
only a few animal mules are known, there is scarcely a genus 
of domesticated plants in which this effect cannot be produced 
by the assistance of man, in placing the pollen of one species 
upon the stigma of another. It is, however, in general only 
between nearly allied species that this intercourse can take 
place : those which are widely different in structure and 
constitution not being capable of any artificial union. Thus 
the different species of Strawberry, of certain tribes of Pe- 
largonium, and of CucLirbitaceae, intermix with abundant 
facility, there being a great accordance between them in 
general structure and constitution ; but no one has ever suc- 
ceeded in compelling the Pear to fertilise the Apple, or the 
Gooseberry the Currant. And as species that are very dis- 
similar appear to have some natural impediment which pre- 
vents their reciprocal fertilisation, so does this obstacle, of 
whatever nature it may be, in general present an insuperable 
bar to the intercourse of different genera. All the stories 
that are current as to the intermixture of Oranges and Pome- 
