The heights of the different piles show the relative frequency of shells having the different numbers of ribs. Seventeen-rib shells were found much more commonly than shells with any other number of ribs. K a line were drawn through the tops of the different shell piles, we should have a frequency curve

FREQUENCY CURVES

relating to the frequency with which different characteristics or items are found repeated is commonly referred to by the biologist as "frequency distribution", and the charts showing frequency are quite often called "distribution charts" or "distribution curves".

In Fig. 140 a frequency diagram is shown at the right by photographing piles of shells arranged so that all shells in any one pile contain the same number of

ribs. The pile of shells at .HBBMSiaKSi^^?jaP**.la^ «■- *-

the left, having the smallest number of ribs, contains but three shells. In the pile of shells at the extreme right, having the highest number of ribs, there is only one shell. The middle pile shows conclusively that the greatest number of the shells have seventeen ribs. There is a fairly large number of shells in the pile for sixteen ribs, and a somewhat greater number of shells in the pile for eighteen ribs. Though it is unfortunate that no horizontal scale or vertical

Fig.

C. B. Davenport, in Popular Science Monthly

141. Forty University Students Arranged in Rows, According to Stature by Inches, as follows: 56 to 57.9, 58 to 59.9, 60 to 61.9, 62 to 63.9, 64 to 65.9, 66 to 67.9, 68 to 69.9, 70 to 71.9