144 THE FLATWORMS— PLATYHELMINTHES 



can be no self-fertilization because there are separate sexes, but some- 

 thing akin to it is found when there is a breeding of closely related 

 individuals. A wise cattle breeder may have a perfectly good bull on 

 his ranch, yet he may go to great lengths to breed some of his cows 

 to other bulls because they may be sisters, daughters, or other close 

 relatives of the bull he owns. He has learned that this outbreeding 

 tends to produce more vigorous offspring than inbreeding. Self- 

 fertilization and inbreeding tend to bring out any latent harmful char- 

 acteristics which may be present, whereas outbreeding tends to cover 

 up these and allow the more beneficial characteristics to be expressed. 



The same principle, of course, applies to human beings, and society 

 has long recognized this fact. Many states have laws against cousin 

 marriages, and incestuous relationships are considered among the worst 

 of all human sins. This does not imply that all children of cousin mar- 

 riages or other relatives will be defective, but only that weak family 

 traits, which might be covered up in a nonrelated marriage, will tend to 

 prevail in the children when both partners carry these same family weak- 

 nesses even though they themselves might not show them. It is quite 

 probable that the vigor of the American nation as a whole can be traced 

 to this same principle. We are somewhat of a melting pot where mem- 

 bers of many races have intermarried, and by this principle we tend to 

 manifest the best qualities of them all. This is sometimes referred to as 

 hybrid vigor and is evident in practically all forms of plant and animal 

 life where the relationships between parents are distant. 



Sexual reproduction seems to be the dominant method in planaria, 

 but it still holds on to an asexual method also. This is by a transverse 

 fission which has a very interesting physiological explanation. As men- 

 tioned under the discussion of the nervous system, the brain, which is at 

 the anterior end of the body, acts as a center of control and coordination 

 of all parts of the body. As the animal grows longer, however, under 

 favorable living conditions, it seems that the posterior end of the body 

 gets so far away from the brain that the brain gradually loses its control 

 over it and it begins to act somewhat independently. A new center 

 of coordination develops at the posterior end and the planaria literally 

 pulls itself in two as a "tug of war" develops between the two ends. 

 The lost portions of each half are then regenerated. This method 

 of reproduction can be prevented if the planaria are kept in containers 

 greased with petroleum jelly. On such a slick surface, there can be 

 no "tug of war" and the two ends are not pulled apart. 



Although planaria shows considerable specialization of tissues, it still 

 has a very great power of regeneration and is an excellent animal to ex- 

 periment with in this connection. If you slit the head down the middle 



