Keeping breeds of horses separate through the generations is difficult and is often carefully managed. There are only one species of the domestic horse. This means that any mare and stallion from any breed can mate and produce offspring that are viable; meaning that the offspring can successfully have their offspring.  As a result, it would be very easy for breeds of horses to become blended through accidental or indiscriminate breeding. If this happened, they would lose the unique attributes that make them well suited for their environment or their role in our society.  There are two ways to prevent this from happening. One is geographical isolation, which is how natural horse breeds developed without human intervention.  Some breeds of horses, such as the Icelandic Horse, are completely isolated from other breeds and have been for thousands of years. There are no other types of a horse on the island of Iceland for the horses to come into contact with. So, the breed stays pure.  The second way is through enforced or artificial isolation of a breed’s genetic pool. This is done using stud books and breed registration.  In summary, to register a horse with a breed managing organization, the horse must be judged against the set breed standards. Horses that do not meet the breed criteria, or who have questionable parentage, may not be accepted into the breed register.  If a stallion or mare has been accepted and registered, it may be matched with an appropriate reproductive partner. The matches are calculated to develop the body and temperament of the breed in certain ways over generations. Proper planning should also keep the gene pool diverse and healthy.