By Martin Stiles -- Left to itself, the species Equus Caballus survived in the wild for centuries, and no doubt could continue to do so. Selecting mates according to the animals' own rules resulted in the development of traits needed for survival in that environment. Domestication imposed different "rules." Today's Thoroughbred is the product of more than 300 years of "improving the breed," and the criteria for selection of Thoroughbred breeding stock probably has not changed much in that time: performance (race record), pedigree, and conformation. As to the relative importance of these three, there is much disagreement. In the choice of racing stock, conformation appears to dominate. (Yearlings, of course, do not have performance records.) But in the choice of breeding stock, conformation seems to take a back seat to the other two. Recently deceased Danehill (by Danzig), sire of more than 2,000 foals, is the leading sire of stakes winners, just ahead of Sadler's Wells (236 and 229). Yet, according to Tony Morris of The Racing Post, he "was noticeably back at the knee...the so-called calf knees have been an identifying characteristic of Danehill's stock." Some would say the success of Mr. Prospector (179 stakes winners) could be viewed in the same light, as further evidence that contemporary breeders look at the paper more critically than at the horse. These two serve to emphasize that the proper test of Thoroughbred quality is on the racetrack, not in the show ring or the library. However, they do not establish that conformation can be disregarded. Selective breeding depends on culling, the systematic rejection of some individuals as breeding candidates. Conscientious culling is a sacrifice, and many temptations are placed in the way of the market breeder confronted by inferior individuals. For example, a foal with severe knock-knees often can have its legs surgically straightened enough to make the yearling bring a good price. But the genes have not been improved by the operation. Similarly, some medications administered to racing animals may mask flaws that can be passed on when that horse or mare is bred. The danger that medication may interfere with improving the breed must surely be one of the main arguments against unrestricted veterinary treatment of racing stock, but that point is seldom mentioned in discussions of the medication problem. Another potential enemy of selective breeding is prosperity. When it is possible to make money from the production of inferior horses, standards for the selection of breeding stock are bound to fall. An increase in the size of our foal crops could be advantageous to those whose livelihoods are linked to serving the horse industry, but it holds some danger for the breed. The size of the foal crop is not regulated by law or determined by any formula accepted by the industry--it is determined by the prospect for profit. The more money perceived by commercial breeders to be available for horse purchases, the larger the foal crop will be. This is a good time to reflect on the size of our foal crops, for a factor may soon be operating to increase them dramatically. Many leaders of the industry have supported the introduction of slot machines and other gaming devices to benefit horse racing. A look at purses offered at venues that already permit slots reveals why. Obviously slots can provide a quick fix for racing economics. Some of the new money will find its way into the hands of buyers of racing stock. Breeders will be prepared to share in the wealth by increasing the size of their broodmare bands. An increase in the Thoroughbred population need not spell doom for the breed, if we keep sight of the need to emphasize quality. Federico Tesio, who bred both Nearco and Ribot from a small group of mares, believed the proper measure of breeding success was performance in the classics, with little concern for the other end of the scale. The Breeders' Cup is basically committed to the same idea. In this writer's opinion, improving the breed would be well served by establishing and maintaining a purse structure that disproportionately rewards the winners of the very best races (grade I), and forbids race-day medication in those races. I think Tesio would approve. Martin Stiles and his wife, Martha, operate Stockwell Farm near Paris, Ky.