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 1 7 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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