54 DARWINISM chap. 



There is here no question of " minute " or " infinitesimal " 

 variation, which many people suppose to be the only kind of 

 variation that exists. It cannot even be called small ; yet 

 from all the evidence we now j^ossess it seems to be the 

 amount which characterises most of the common species of 

 birds. 



It maybe said, however, that these are the extreme variations, 

 and only occur in one or two individuals, while the great 

 majority exhibit little or no difference. Other diagrams will 

 show that this is not the case ; but even if it were so, it would 

 be no objection at all, because these are the extremes among 

 thirty specimens only. We may safely assume that these thirty 

 specimens, taken by chance, are not, in the case of all these 

 species, exceptional lots, and therefore we might expect at least 

 two similarly varying specimens in each additional thirty. But 

 the number of individuals, even in a very rare sj^ecies, is 

 probably thirty thousand or more, and in a common species 

 thirty, or even three hundred, millions. Even one individual 

 in each thirty, varying to the amount shown in the diagram, 

 would give at least a million in the total population of any 

 common bird, and among this million many would vary much 

 more than the extreme among thirty only. We should thus 

 have a vast body of individuals varying to a large extent in 

 the length of the wings and tail, and offering ample material 

 for the modification of these organs by natural selection. We 

 "vvill now proceed to show that other parts of the body vary, 

 simultaneously, but independently, to an equal amount. 



The first bird taken is the common Bob-o-link or Rice-bird 

 (Dolichonyx oryzivorus), and the Diagram, Fig. 4, exhibits the 

 variations of seven important characters in twenty male adult 

 specimens.^ These characters are — the lengths of the body, 

 wing, tail, tarsus, middle toe, outer toe, and hind toe, being as 

 many as can be conveniently exhibited in one diagram. The 

 length of the body is not given by Mr. Allen, but as it forms 

 a convenient standard of comparison, it has been obtained by 

 deducting the length of the tail from the total length of the 

 birds as given by him. The diagram has been constructed 

 as follows : — The twenty specimens are first arranged in a 

 series according to the body-lengths (which may be con- 

 ^ See Table I, p. 211, of Allen's Winter Birds of Florida. 



