5o6 BIOLOGY AND HUMAN LIFE 



better than their fellows? Will the seeds of the tallest plants 

 always yield a new generation of the same qualities? Expe- 

 rience has shown that we cannot depend upon their doing so 

 unless the superior conditions are also repeated. If the condi- 

 tions for the next generation are only average or inferior, the 

 good seed will not insure a superior growth. If we select varia- 

 tions from the average which depend upon favorable conditions, 

 we have to continue selection, generation after generation. 



368. Instability of hybrids. By crossing two varieties of 

 plums or apples or potatoes or cabbages or squashes we can 

 often get a crop of individuals (hybrids) that differ from either 

 parent in a striking way; but if we plant the seeds of one of 

 these improved or interesting hybrids, the following generation 

 will show an even greater assortment of unusual individuals 

 than we had among the hybrids or else will show an assortment 

 that is more like the grandparents. It has been known among 

 breeders of plants (as well as of animals) that the descendants 

 of hybrids tend either to "break up" or to "throw back" to the 

 ancestral forms. For this reason it is not possible to rely upon 

 hybrid seed, for there is no telling what it will develop into. If 

 you had a hybrid apple that combined the keeping qualities of 

 one brand of winter apple with the large size or fine flavor of 

 another variety, you would want to multiply this new type ; but 

 if you depended upon the seeds, you would be disappointed. 



369. Preserving hybrids. If you knew which tree produced 

 the new kind of apple, you could multiply the number of 

 branches bearing such hybrids without using the seeds at all. 

 By grafting twigs of this tree upon the stocks of other vigorous 

 apple trees you could in a few years transform an orchard bear- 

 ing an old type of apple into one bearing the new type, without 

 ever using any seeds. This vegetative propagation not only in- 

 sures a greater degree of uniformity in the product but also saves 

 much time, since fruit can thus be obtained without waiting for 

 plants to grow from seeds until they are old enough to bear. 



The same principle applies to raising improved potatoes, 

 for example, except that here we use, not grafting, but bud- 



