Almost every article about guppies tells us that all the males are so unlike one another that no two could be found which could not be told apart. These authors could never have seen inbred guppies. Among mine are three strains which are the tenth consecutive generation of brother and sister mating. The males are now so similar that they look as much alike as twins.
 Such guppies are the only safe kind to use in studying heredity; practically all others are unreliable for it is too easy to draw unwarranted conclusions when using them.
 And guppies are unusually rugged in this regard, standing inbreeding remarkably well. The same precautions must be taken with them as with any other species. From each generation, the largest and the most vigorous must be chosen to be parents of the next. Inbreeding simply concentrates the characteristics. If there are a lot of inferior features they will become congregated in a single fish and probably produce an inferior fish. By the same token, the good are also congregated. Each generation is a little less prolific. In my inbred guppies, each generation was a little smaller and a little less vigorous down to the fifth and from there on, an improvement occurred. My tenth generation inbreds have better guppy characteristics than the fifth, but not as beautiful as the one with which I started.
 Inbreeding - Outbreeding. The best corn, hogs, chickens and now guppies are the result of outbreeding two unrelated inbred strains-inbred for seven or more generations. The flashy, large and most sensational guppies of the future will doubtless be of this kind. Unfortunately such guppies are of little value for breeding purposes because their offspring are decidedly downward in the scale. The practice rests on hybrid vigor-heterosis. The system has the advantage of preventing competition.
 Line Breeding. Mating closely related animals is line breeding. The relationship of line breeding usually considers relatives further removed by inheritance than first cousins. Line breeding is safe and has none of the disadvantages of inbreeding. It is used principally in conserving valued characteristics.