September 20, 1912] 



SCIENCE 



383 



nippled ewes that have all six nipples in a 

 functional condition. "We fully expect an in- 

 crease in the percentage of six-nippled lambs 

 born on Beinn Bhreagh next year, in spite of 

 the fact of the large amount of alien blood in 

 the flock. The new blood introduced has not 

 diminished the proportion of six-nippled 

 lambs born on the place, while it has resulted 

 in increased vigor and strength in the flock 

 as a whole. The physique has been improved 

 to such an extent that there are now no better 

 nor finer sheep than those to be found on 

 Beinn Bhreagh. 



A Discovery Relating to Color. — During 

 the course of our breeding experiments a very 

 curious fact made its appearance. It is well 

 known that when white ewes are mated with 

 white rams, black lambs occasionally appear 

 among the offspring; but it has not hitherto 

 been known that when black ewes are mated 

 with black rams, the offspring are always 

 ilach. 



This fact was discovered by Dr. Davenport, 

 Director of the Carnegie Institution's Station 

 for Experimental Evolution at Long Island, 

 N. Y., from a study of the records of the 

 Beinn Bhreagh flock; and his conclusions 

 have since been amply verified at Beinn 

 Bhreagh and elsewhere. 



Production of Twins. — The experience of 

 the past has shown that twin lambs are 

 usually the produce of old ewes; and the fact 

 that 36 per cent, of the lambs born on Beinn 

 Bhreagh are twins is very encouraging when 

 we consider that they are the produce of 

 young ewes. 



We are getting twins from mothers that 

 were only one year old, and two years old, at 

 the time of the birth of their lambs; and 60 

 per cent, of the lambs born this spring from 

 three-year-old ewes were twins. 



We have only one old sheep on the place, 

 No. 1505. She was seven years old this spring 

 and has given us eleven lambs to date. She 

 started out with twins when she was only a 

 lamb herself (one year old) and has given us 

 three sets of twins since then, besides three 

 single lambs. 



The indications are that our six-nippled 



stock will ultimately turn out to be twin bear- 

 ers, as a rule, when they become fully mature. 

 In the meantime the question is : What can 

 we do to favor the production of twins in our 

 flock? 



1. One point of importance will be to see 

 that twin ewes are mated with twin rams. 



2. Twin-bearing ewes are, on the average, 

 much heavier than single-bearing ewes. We 

 would therefore favor the production of twins 

 by weeding out from the flock, while still 

 young, ewes that are markedly under the 

 average weight of the ewes of their age. 



3. When our flock of adult ewes is large 

 enough to permit of selection it might be well 

 to discard ewes at the age of three years, at all 

 events at four, if they have never given us 

 twins ; so that the bulk of our fully mature fe- 

 males should ultimately be composed of ewes 

 that have produced twins when young. 



These points relate to efforts to increase the 

 hereditary tendency to the production of 

 twins; but there are other points relating to 

 environment which are also of importance. 



Condition of Nutrition Important. — The 

 records of weight that have been preserved at 

 Beinn Bhreagh seem to indicate that the con- 

 dition of nutrition of the mother at the time 

 of mating has an important influence upon 

 the conception of twins. Our records show 

 that the twin-bearing ewes increase in weight 

 as the mating period approaches; and that 

 mating occurs when the ewes are in prime 

 physical condition. 



This probably explains the curious fact that 

 the ewes, which are mated in October, have a 

 much larger proportion of twin lambs than 

 those which are mated later on in the breed- 

 ing season; thus verifying the old saying of 

 the farmers here that " March Iambs bring 

 twins." The pasture is at its best in October. 

 It begins to fail in November, and by Decem- 

 ber there is very little left. Our records show 

 that when ewes are mated in October the 

 proportion of twins born is larger than when 

 mating occurs in November, and that very few 

 of the ewes mated in December have twins. 



In order to favor the production of twins it 

 is therefore necessary, or at least advisable. 



