286 
PHYSIOLOGY. 
BOOK II. 
they have the power of parting with their fertilising principle 
in another manner. 
One of the most curious consequences of the presence of 
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 similiar 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 by seed ; or, should it not be 
absolutely sterile, it will become so in the second, third, or, 
very rarely, fourth generation. 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 
appeal's 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 
Pelargonium, and of Cucurbitaceae, intermix with the greatest 
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, nor the 
gooseberry the currant. And as species that are very dis- 
similar appear to have some natural impediment which pre- 
