INBREEDING AND CROSSBREEDING 2!29 



many plants of floral structures, which insure crossing 

 through the agency of insects or of the wind. 



In animals the facts as regards close fertilization are similar 

 to those just described for plants. Some animals seem to be 

 indifferent to close breeding, others will not tolerate it. Some 

 hermaphroditic animals (those which produce both eggs and 

 sperm) are regularly seK-fertilized. Such is the case, for 

 example, with many parasitic flatworms. In other cases 

 seK-fertilization is disadvantageous. One such case I was 

 able to point out some twenty years ago, in the case of a sea- 

 squirt or tunicate, Ciona. The same individual of Ciona 

 produces and discharges simultaneously both eggs and sperm, 

 yet the eggs are rarely seK-fertilized, for if self-fertilization is 

 enforced by isolation of an individual, or if seK-fertilization is 

 brought about artificially by removing the eggs and sperm 

 from the body of the parent and mixing them in sea water, 

 very few of the eggs develop, — less than 10 per cent. But if 

 the eggs of one individual be mingled with the sperm of any 

 other individual whatever, practically all of the eggs are 

 fertilized and develop. 



In plants much attention has been given to the problem of 

 self -sterility by East, Stout, Dorsey, and others. The case 

 of native American plums is as simple as any. All varieties 

 investigated by Dorsey were found to be self-sterile. If self- 

 pollinated, they set no fruit, either because the pollen grains 

 fail to germinate or because the pollen tubes, if formed, grow 

 too slowly to reach and fertilize the ovules. Not only are 

 all varieties self -sterile, some are also cross-sterile, i. e., sterile 

 when crossed with each other. It is probable that such 

 varieties have inherited a similar genetic constitution, so 

 that the pollen of one reacts toward the pistil of another as 

 toward pistils of its own plant. In support of this view it 

 may be said that East and Park found that the F. phmts 

 produced by crossing Nicotiana Forgeiiana with N. data fell 

 into four groups, all the plants in each group being mutually 

 cross-sterile but fertile with any plant of the other three 

 groups. The obvious conclusion is that the plants of each 



