456 



POPULAR SCIENCE MONTHLY. 



possibly not far from 96. Likewise the Littorinas from South Kin- 

 cardineshire, Scotland, have a modal index of 88 and a positive skew- 

 ness of 0.065; while those of the Hnniber, having a mode of 91, have a 

 skewness of -|- 0.048. These figures suggest an ancestral index of 

 about 97, or about the same as before. The fonn of the frequency 

 polygon may thus enable us to infer the ancestral condition of a race 

 or species and may consequently help us to get at the history of the 

 race. 



Skewness may, as we have seen, depart to any extent from a nearly 

 symmetrical condition. The extreme case occurs when the mode lies 

 at one end of the range. This case is sometimes found among plants. 

 It indicates that the group has in respect to the character in question 

 reached an extreme condition (Fig. 10). 



Complex frequency polygons have various interpretations. We 

 have already seen that one of these interpretations at least is a splitting 

 of one race into two. Another kind of complex polygon is due to 



? 2 3 4 5 6 7 8 9 10 II 12 15 1* IS fm. 



Fui.l2. Complex curve of length in cm. of fasciated stem of 146 individuals of Crepis 

 biennis derived from fasciated ancestors. one individual of 19 centimeters omitted. 

 From de Vries, "95, Bulletin Scientifique, Tome 27, p. 397. Ordinates, number of indi- 

 viduals ; ABsciss.iJ, length of fascia rED stem. 



differences of age. Suppose an animal that breeds at one restricted 

 season of the year and that annually nearly doubles in size. If we 

 inake a collection of a lot of these animals from a place at one time, 

 we shall include individuals of different ages such as, for instance, six 

 months, one year and six months, two years and six months, and so 



