Some confusion seems to exist among many people as to the relative usefulness of the various tools available to decide if a horse is or is not of Spanish descent. This problem is especially complicated in the North American situation since the goal is to re-isolate a genetic resource that was nearly crossed out of existence. Since a lot of crossbreeding occurred on the original Spanish base, the overall goal of conservers of the Spanish of the Spanish Mustang has been to eliminate crossbred horses while trying to include all purely Spanish horses. The three main tools for evaluating horses are the history behind the individual horse, the appearance of the horse, and blood type of the horse. None of these is a perfect mechanism in all cases, but some common sense will help in eliminating the errors of including non-Spanish horses, and excluding Spanish horses. Both errors occur to the detriment of the breed: one error allows outside blood to be unwittingly used, the other eliminates the use of perfectly good and useful genetic material. Both errors should be avoided.

The evaluation of the history and external appearance can be relatively cheaply and quickly done, so these are the two criteria to start with when evaluating most herds or individual horses. The history can point to either a prolonged existence as an isolated group, or to repeated crosses. The history desired is that of an isolated group. Of course, the type of horses that went into the isolation is as important as the fact of the isolation. If crossbreds went into it, crossbreds come out. That is why history alone cannot fully determine if a group is Spanish or not, but rather must be considered with respect to the external appearance, and sometimes with the blood type as well.

The history must be evaluated carefully. Clean histories that are well documented are very unlikely to occur at this late date. Histories are also often tailored to the audience, or by the teller. With feral horses especially, if the person who is asked the history wants the horses removed, then he is very likely to indicate that they were crossbred originally and crossbred since. If the investigator is very interested in the horses, and the teller begins to smell money, then the history may change to one of isolation and pure Spanish breeding. This is where good questioning can help. The end result is that it is difficult to really pin down the history on many herds of horses for a variety of reasons. To further complicate the story, there are rarely any external sources to corroborate or document the history.

External appearance also needs to be evaluated. One problem with this is that the original horses from Spain varied as to both quality and type. Some were very poor quality horses, and Spanish type itself varies even among the high quality individuals. The registries in North America have concentrated on the type that is most unique among Spanish horses. This has some merit, since some of the other purebred Spanish types are fairly easily confused with crossbred horses. In a herd of impeccable history and breeding, these less unique types are no indication of non-Spanish breeding. In herds of less documentation, they are fairly risky to include, since they might point to crossbreeding. The point of bringing up these less unique Spanish types that have been excluded by the North American registries is not to suggest that they should be included, but rather to stress that their presence in a herd does not mean that the rest of the herd should be excluded by virtue of being crossbred. The horses of the less unique types could even participate in a conservation breeding program since many of them will produce more typical horses that should participate directly in registered herds. Though of less unique external type, they are still useful to the conservation of Spanish horses in North America.

The two criteria discussed so far are very removed from strict science: The historical investigation is frequently blurry or questionable, and the evaluation of appearance can generally pick out the very typical (as defined by the registries) horses, but cannot tell certain Spanish types from more common crossbred horses.

Blood typing is thought by many to hold the final answer to this dilemma and is seen as an infallible scientific answer to the problem. Unfortunately it is not, but merely adds a useful third leg to the evaluation of the history and external appearance.

Blood typing in horses and other animals involves a peek into the variability of certain substances in the blood. Each different substance is generally referred to as a "system". There are many such systems, including both enzymes and cellular surfaces antigens (proteins), and about seventeen of these are useful in horses. To be useful, a system must be capable of some sort of characterization (the two methods are complicated, and involve either immunologic or electrophoretic techniques--these details are less important than the more easily understood fact that they can be reliably characterized as unique). For any system to be useful in this endeavor, it must occur in at least two different forms. Each form within a system is usually called a marker, since it can be consistently and reliably recognized. The point of blood typing is to demonstrate differences between horses, and if all markers of one system were identical in all horses then obviously that system cannot be used for the purpose of demonstrating differences. Some of the systems have many different forms (each representing a marker), which makes these particular systems very useful for distinguishing among horses.

The fact of many different systems, each with different markers, is the basis of being able to distinguish some individuality of horses by blood typing. One common use of blood typing is as a check on parentage. Each system occurs as a pair of markers for that system for each horse. One of the markers comes from the sire; one from the dam. This is simple genetics. The foal cannot pick up any marker that is not present in its parents, and it must inherit one marker of each pair from each parent. Blood typing for parentage works extremely well for horses due to the number of systems and the fact that within each there is a pair of markers for each horse. The result of this is that most horses are unique as far as blood type goes, although occasional pairs of horses may be identical for blood type.

The use of blood typing for determination of breed is different than the use for confirmation of parentage. Breeds vary due to reproductive isolation. Put another, Percherons and Shetlands are different in part due to a long, long history of never having been mated together. Over a long period of time, the result is that the blood type markers ae occasionally limited to certain breeds, or more commonly are present in many breeds but in different frequencies in each of them. The story gets somewhat complicated at this point. Most horse breeding over the last centuries has involved cross breeding of different types and styles of horses. The result is that most breeds share many of the markers of the blood type systems. All that can distinguish these breeds from one another is the frequency of the markers within the group or breed and not their presence or absence.

There are a few blood type markers which are unique to some breeds. These unique markers can help in some instances, but not in all. Two examples will show the pitfalls that can occur by relying on blood type alone.

First, many horses fail to pick up blood type markers that are unique to any breed or breed group. So, some pure Spanish horses will have an essentially "generic horse" blood type. Nothing unique, no clues about the origin of the horse. Such a blood type could occur in any of a huge number of purebred horses from different breeds. The lack of a unique blood type marker does not make these horses any less purebred or typical than those horses that do have them. In these cases the history and the external appearance are the important portions of the evaluation, since the blood type does not help at all.

The opposite problem can also occur. A detailed example will serve to contrast a purebred horse with a generic blood type from a cross bred horse with Spanish markers. There are many, many markers that point to a Spanish origin, but are shared among many breeds. There are only two that (so far as is known) are unique to Spanish breeds. These two involve involve different systems, so can both be passed to a foal from one parent (contrast this with the case where they are both markers in the same system, in which case only one could pass). We will start with a purely Spanish stallion that has been fortunate enough to have both of these markers. He is for some reason bred to a herd of draft mares. One fourth of his foals pick up both Spanish markers. If a colt with both of these is in turn used on draft mares, then one fourth of those likewise get both markers. The end result is a grade draft horse, probably betraying nothing of its Spanish ancestry in looks or performance, but having uniquely Spanish markers in the blood type. This horse by blood type alone would be considered more Spanish than the purebred Spanish horse with a generic horse blood type in the earlier example.

A third phenomenon can help in the evaluation of blood type. Other breeds and breed groups do have a few unique markers in addition to the ones shared by a wide variety of horse breeds. If a horse has one of these unique blood type markers from another breed source then this can betray a crossbred origin. Unfortunately these are very rare, and consequently the horses that have them are likewise rare.

So, it is possible to breed non-Spanish horses with Spanish blood type markers, and equally possible to breed Spanish horses lacking Spanish blood type markers. This is an important lesson, even if from an extreme and hypothetical example . The important point is that it is impossible to rely on any one of the three evaluation criteria alone: history, external appearance, and blood type. The history can be used to eliminate horses that are likely to be crossbred. The external appearance can be used to substantiate the history, or to guide the evaluator either towards or away from deciding on a Spanish type horse or herd. External evaluators do need to be aware, though, that purebred Spanish type does vary more than what is included in the registries. The blood typing can be used to further substantiate the history and external appearance. In cases where one of the criteria is missing, the other two can point the way. For example, a horse with a very typical Spanish appearance and a very typical Spanish blood type is very likely a good horse to register, even in the absence of a detailed history.

The bottom line is that common sense needs to be used when evaluating horses by any method. All the tools available will never replace common sense or experience, nor should they be expected to. The goal for the registries should be to try to include all the purebred Colonial Spanish horses while including none that are not purebred, and that is a very difficult line to walk.