The two terms _continuous_ and _discontinuous variation_ refer to the succession or inheritance of the variations rather than to the actual conditions amongst a group of individuals living at the same time; but this distinction has only a subordinate value. The term _fluctuating_, or _individual variation_, expresses more nearly the conditions of the individuals of a species at any one time, and the continuation of this sort of difference is the continuous variation spoken of above. The discontinuous variations are probably of the same nature as those that have been called mutations, and what Darwin sometimes called sports, or single variations, or definite variations. Continuous Variation If we examine a number of individuals of the same species, we find that no two of them are exactly alike in all particulars. If, however, we arrange them according to some one character, for example, according to the height, we find that there is a gradation more or less perfect from one end of the series to the other. Thus, if we were to take at random a hundred men, and stand them in line arranged according to their height, the tops of their heads, if joined, would form a nearly continuous line; the line will, of course, incline downward from the tallest to the shortest man. This illustrates individual variation. An arrangement of this kind fails to bring out one of the most important facts connected with individual differences. If the line is more carefully examined, it will be found that somewhere near the middle the men are much more nearly of the same height, or rather there are more men having about the same height than there are near the ends of the line. Another arrangement will bring this out better. If we stand in a line all the men from 60 to 61.9 inches, and in another parallel line all those between 62 and 63.9, then those between 64 and 65.9, then between 66 and 67.9 inches in__ height, etc., it will be found that there are more men in some of these lines than in others. The longest line will be that containing the men of about 65 inches; the two lines formed out of men on each side of this one will contain somewhat fewer men, and the next ones fewer still, and so on. If we looked at our new group of men from above, we should have a figure triangular in outline, the so-called frequency polygon, Figure 3 B. With a larger amount of data of this sort it is possible to construct a curve, the curve of frequency, Figure 3 A. In order to obtain this curve of frequency, it is of course not necessary to actually put the individuals in line, but the curve can be drawn on paper from the measurements. We sort out the measurements into classes as in the case given above. The classes are laid off at regular intervals along a base-line by placing points at definite intervals. Perpendiculars are then erected at each point, the height of each being proportional to the frequency with which each class occurs. If now we