134 REPRODUCTION 



fertilization described in the preceding section, except that 

 the gametes are distinctly different, are formed in different 

 parts of the body, and their union introduces a little more 

 possibility of newness and variety in the result. 



4. Union of Dissimilar Gametes from Different Parents. 



While self-fertilization is possible in the case of some of 

 the animals and plants which produce both kinds of 

 gametes in the same parent, it is not the rule. In a form 

 such as the earthworm or snail there are interesting 

 adjustments that tend to prevent the sperm of one animal 

 reaching and fertilizing the eggs of the same animal. In 

 both of these cases, when copulation between two animals 

 takes place, each animal transfers sperms to the other, and 

 later the sperms are brought into union with eggs of other 

 parentage. This is cross-fertilization. But the most 

 common and sure kind of cross-fertilization in the animal 

 kingdom is in the mating of parents which produce only 

 one kind of gamete, exclusively male parents mating with 

 exclusively female parents. In such case cross-fertilization 

 is insured by making self-fertilization impossible. 

 Separate maleness and femaleness is a device which 

 insures cross-fertilization. 



In the common seed-plants we have something quite 

 similar. Most of the plants produce both kinds of spores 

 in the same flower, and if the sperms produced by the 

 pollen of a flower fertilize the eggs produced by the 

 gametophytes in the ovary of the same flower, we might 

 call it self-fertilization. We call it self-pollination, when 

 the pollen of one flower acts on the pistil of the same 

 flower. Self-pollination, however, seems to be the 

 exception in higher plants rather than the rule. Just as 

 in animals, there are many devices which prevent self- 

 fertilization, and secure cross-fertilization. In plants 

 there is a most wonderful series of adaptations tending to 

 insure cross-pollination and to discourage self-pollination. 

 Sometimes, as in the gametes in animals which have 



