September 22, 1898] 



NA TURE 



503 



In the next case, this is not true. 



The diagram (Fig. 4) represents the number of female swine, 

 out of a batch of two thousand examined in Chicago, which 

 have a given number of Miillerian glands in the right fore leg. 



amount of possible change is greater in one direction than in 

 the other. 



Now let us pass on to another example. 



Table III. shows the variation in the number of petals in a 

 race of buttercups studied 

 by Prof, de Vries. You 

 see that the most frequent 

 50 



720 730 



Fig. 3. — Diagram sh 1 



ving the magnitude of the antero-lateral margin (i 

 999 female shore-crabs from Naples. 



The distrilnition is much more skew than in the case of the crabs, 

 and you see again the very beautiful way in which Prof. Pearson's 

 curve expresses it. You see that the range of variation is much 

 greater on one side of the 

 mean than on the other ; 

 and the selective destruction 

 necessary in ordt-r to raise 

 the mean number of glands 

 by one would be very different 

 from the amount of destruc- 

 tion necessary in order to 

 lower the mean by one. 40. 

 Further, the mean number 

 of glands in these pigs is 3^ ; 

 the number which occurs 



ftenest, the " modal "num- 

 'er as Prof. l*earson calls 

 it,' is three. Now it is im- 

 possible to lower this num- 

 ber till it is less than o, so 

 that it can only be diminished 

 by three ; but it is conceiv- 

 able that it should be in- 

 creased by more than three. 

 So that the amount of .se- 

 lective destruction required 2 > 

 in order to change either 

 the mean or the modal cha- 

 racter of these pigs in one 

 direction, would be greater 

 than the amount required, 

 in order to produce a change 

 of equal magnitude in the i<. 

 opposite direction, and the 



1 All attempts to confine the 

 word "average" to the most fre- 

 quently occurring magniiude, and 

 the word "mean" to the arith- 

 metic mean of the series, have 

 failed to secure support. There- 

 fore Prof. Pearson's proposal to 

 call the value which ^ occurs 

 oftenest the " mode " is very 

 useful. 



780 790 800 



n terms of carapace-length) in 



number of petals is five, and 



that no buttercups whatever 



have less than five petals, though a considerable number have 



more than five ; and here again you see the way in which Prof 



Pearson's formula fits the observations. 



NO. 1508, VOL. 58] 



Fig. 4.— Diagram showing the number of Miiller's glands in each of ;ooo f.male 



