1. A study has been made of the degree of inbreeding in the Ayrshire breed of cattle.

2. The coefficient of inbreeding for the whole breed, as calculated by Wright's “Approximate (or Short) Method,” showed a progressive increase from nil in 1877 to a mean value of 5·3 in 1927.

3. No difference was found in the coefficient of inbreeding as between registered bulls and cows.

4. A large proportion of the inbreeding was traced to two foundation sires, Burnhouses (8) and Hover-a-Blink of Drumjoan (892).

5. Using Wright's “Long Method,” it was found that high milk-yielding cows showed a markedly lower coefficient of inbreeding than the breed average. On the other hand, when the coefficient of inbreeding of high milk-yielding cows (over 1000 gallons) was compared with that of cows giving average yields (under 800 gallons) it appeared that inbreeding itself had no detrimental effect on milk yield.

6. In 1923, 39 per cent, of the inbreeding ties of high milk-yielding cows were traced to the Drumjoan herd, as against the next highest herd which had only 6·37 per cent, of ties. Of the total coefficient of inbreeding the Drumjoan herd was responsible for 18·36 per cent., whereas the next highest herd was only responsible for 8·35 per cent. The 1928 results also demonstrated the marked influence of the Drumjoan herd. Practically 50 percent, of these ties were traced to animals related to the foundation sire, Hover-a-Blink of Drumjoan. These results demonstrate the profound influence which one herd, and indeed one bull, may exert in the genetic construction of a recently established breed.

7. It is pointed out that Wright's “Long Method,” when applied to breeds of relatively recent origin, gives far more reliable results than his “Short Method.”