450 



SCIENCE 



[N. S. Vol. XXVII. No. 690 



discussed are from a single breed of fowls, 

 Barred Plymouth Rocks. They are based 

 on returns from several thousand hens. 

 It is hoped that a detailed account of the 

 results will be published shortly. 



Variation in Annual Egg Production. — 

 If frequency distributions of individual 

 variation in total annual egg production 

 be made, they are found: (1) to be uni- 

 modal; (2) to exhibit negative skewness in 

 all but a few cases. In these cases the 

 variation curve is symmetrical; (3) to con- 

 form to Pearson's Type I curve when un- 

 symmetrical, and to his Type II curve 

 when symmetrical: (4) to exhibit a high 

 relative degree of variation (coefficient of 

 variation = 35 per cent. ca.). 



Comparative Distribution of Birds and 

 of Eggs produced.— In consequence of the 

 skewness of the variation curves it results 

 that 50 per cent, of the total number of 

 birds produce from 60 to 76 per cent, 

 of the total number of eggs. This is of 

 practical consequence in breeding, since it 

 means that a random sample of eggs will 

 not represent a random sample of the birds 

 which produced them, but instead will in- 

 clude an unduly large proportion of heavy 

 layers. 



Monthly Distribution of Egg Produc- 

 tion. — The maximum mean egg production 

 is in March or April. Secondary maxima 

 occur in January, June and September. 

 These maxima are entirely independent of 

 the total egg production, appearing in the 

 records of the lowest as well as the highest 

 egg producers. If the difference between 

 the heaviest and lightest layers in respect 

 to the mean egg production for each 

 month in the year be taken, it is found 

 to be smallest in March, April and May, 

 largest in the late summer and early 

 autumn months. The individual variation 

 in egg production (measured eitlier by 

 standard deviation or coefficient of varia- 



tion) is greatest in those months when the 

 mean production is least, and vice versa. 

 There is no sensible correlation between 

 the egg production in the pullet year and 

 that in the second laying year. 



Some Effects of Differences of Tempera- 

 ture and Humidity upon the Post-natal 

 Development of the Mouse: F. B. Sum- 

 ner. 



White mice which were transferred at 

 the age of two to four days to two rooms 

 having mean temperatures of about 45° 

 and 75° F. respectively (the relative 

 humidity being about twice as great in 

 the former as in the latter) were found 

 to exhibit great differences in tail length 

 when measured at the age of 42 days. 

 Although the mean weight of those kept 

 in the warm room (51 individuals) was 

 less than 6/10 gm. greater than that of 

 the cold room lot (44 individuals), the 

 mean tail length was 69.3 mm. in one case, 

 and 52 mm. in the other, i. e., there was a 

 difference of over 33 per cent. Moreover, 

 the shortest tail in the warm-room lot was 

 longer than the longest tail in the cold- 

 room lot. 



In a previous experiment, in the course 

 of which 20 mice in each lot were sub- 

 jected to conditions similar to those stated 

 above for a period of from 83 to 106 days, 

 commencing with the fourth week after 

 birth, the difference in tail length was 

 7.1 mm., or 11 per cent, of the tail length 

 of the cold-room lot. On the other hand, 

 the mean weight and the mean body length 

 (distance from snout to anus) in the two 

 lots was abnost identical. In this experi- 

 ment, the males of the cold-room series ( 13 

 individuals) were found to have an aver- 

 age of 305 hairs in a given unit of skin 

 surface (2 disks, each 1.5 mm. in diam- 

 eter), while those of the warm-room lot 

 had, on the average, 267 hairs in the same 

 area of skin. The variability in each case 



