CHAPTER TWENTY-FIVE. 

 CROSSING BREEDS. 



1. Reproduction without Union. In reproduction such 

 as fission in bacteria, budding in yeast, spore production 

 in molds, and stolons, buds, grafts, or tubers in the higher 

 plants, only one parent takes part in the formation of the 

 new individual. In such cases the offspring is likely to be 

 just like the single parent, because it is simply a portion 

 of it. The single parent stamps its nature most strongly 

 upon the offspring. It is frequently found that such stock 

 gradually "runs down" if continued indefinitely, as in the 

 case of Irish potatoes. 



2. Union of Similar Gametes from the Same Parent. 

 In cases where one kind of gamete is formed and the union 

 takes place between cells that have the same parent, we 

 have two nuclei taking part in the union, but they, having 

 the same ancestry, are likely to be very similar. Never- 

 theless it appears that such a union as this tends to 

 restore and strengthen the vitality of the embryo, as 

 compared with organisms in which no union occurs. This 

 is known as self-fertilization. 



3. Union of Unlike Gametes from the Same Parent. We 

 have seen that there are a good many hermaphrodite 

 parents, in which one parent produces both eggs and 

 sperm. The gametes are different, but the parent is one. 

 This is found, for example, in tapeworms, in earthworms, 

 in snails, in many plants. Doubtless self-fertilization 

 occurs in many of these cases. It is similar to the self- 



133 



