22 Mr. Francis Galton on 



A would in this case symbolise large areas, and Z small 

 ones. Or the subject of experiment might be the depth 

 of the general tint. Or, again, it might be the greater 

 or less acuteness of the angle of the wing. Any variable 

 characteristic that exists in both sexes and in all 

 individuals can be treated in this way. 



As there is no difficulty in treating the two sexes on 

 equal terms in statistical inquiries by first transmuting 

 all female measurements to their male equivalents [in 

 human stature this is effected by adding one-twelfth or 

 thereabouts; I use the multiplies of 1*08], so probably 

 the different broods of dimorphous moths may be 

 rendered comparable in the same way, and it may be 

 found unnecessary in Selenia illustraria to confine the 

 comparisons between spring-brood and spring-broods, 

 and between autumn-brood and autumn-broods respec- 

 tively. Thus the advantage of double-broodness, in 

 giving two steps a year instead of one, will probably 

 not be lost by experimenting on a dimorphous species. 



Measurement. — The wing-length is to be measured 

 from the root of the wing to its tip (that is, from a to b 

 in the fig.), a has to be estimated from 

 the run of the upper and lower mar- 

 gins, and can be determined with fair 

 precision, b is very well-defined in a 

 normal wing. Should, however, the tip 

 be injured, the measurement is to be made from a to c, 

 and stated accordingly. Again, if both b and c are 

 injured, measure from a to d, and state so. 



In performing the measurement with a pair of com- 

 passes (or other scale) it is necessary to bring the points 

 very close to the moth, else a slight change in the posi- 

 tion of the eye will give discordant readings ; and if the 

 eye is not moved at all the readings will be sensibly too 

 small. A magnifying-glass is desirable. For measuring 

 living but sluggish moths, I have contrived a pair of 

 compasses that work well and quickly when making 

 the ordinary measurements, and which carry a magni- 

 fying-glass. I will not, however, now give a drawing of 

 it, because I want to assure myself of the handiest form 

 of the instrument before doing so. Its principle is this. 

 Imagine a pair of fine-pointed scissors with very long 

 arms, — say five times as long as the blades ; the points 

 of the blades represent the compass-points. At the end 



