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 knoAvn 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 uniform!}' 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 off'spring 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 

 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- 



