The Making of Species 
tinuous variations, and as some writers do not 
appear to realise wherein lies the essential 
difference between the two kinds of variation, 
we will, at the risk of appearing tedious, give 
a further illustration. Let A be a species of bird 
of which the average length of the wing is 
20 inches, and let us suppose that individuals 
belonging to that species occur in which the 
length of the wing varies as much as 3 inches 
each side of the mean ; thus it is possible to find 
individuals of this species with a wing as short as 
17 inches, or as long as 23 inches. Let B be 
another species of which the average length of 
the wing is 17 inches, and let us suppose that a 
3-inch variation on each side of the mean be 
found to occur. Individuals belonging to species 
B will occur which have a wing as short as 
14 inches, or as long as 20 inches. Thus some 
individuals of the short-winged species will have 
longer wings than certain individuals of the long- 
winged species. Similarly, certain individuals 
of a species which display a mutation may show 
less deviation from the mean than some indi- 
viduals showing a very pronounced fluctuating 
variation. In other words, even as by measuring 
the length of wing in the above example it was 
not always possible to say whether a given indi- 
vidual belonged to species A or B, so is it not 
always possible to say by looking at an individual 
that shows a considerable departure from the 
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