NATURAL SELECTION 9 



that almost all sorts of variations are always present among 

 the individuals of every species. \Ye have taken illustrations 

 from the plants ; of course the same phenomena are found 

 among animals (Plate 3). 



Not only is variation universal, affecting all organisms 



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and all parts of every organism ; we find also that the degree 

 of divergence is really very great. In some of our common 

 birds, for example, the length of wing varies to the extent of 

 one quarter of the average for the species. So also with the 

 length of tail, the proportion between length of wing and 

 length of tail, the size of beak, the proportions of the legs, 

 feet, toes, and claws, and many other characters. Mr. J. A. 

 Allen, in his memoir On the J\faniuials and M^intcr Birds 

 of East Florida, says, " The facts of the case show that a 

 variation of from fifteen to twenty per cent in general size, 

 and an equal degree of variation in the relative size of 

 different parts, may be ordinarily expected among specimens 

 of the same species and sex, taken at the same locality, while 

 in some cases the variation is even greater than this." 



Animals and plants do not all show an equal amount of 

 variation. Amonsr animals the domestic o;oose is a o-ood 



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example of a species in which variation is comparatively 

 slio-ht. Partly as a result of this stability, domestication has 



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resulted in the establishment of but few breeds of geese. 

 But even in those species of animals and plants in which 

 there is the least variation the differences bet\veen individ- 

 uals are still readily noticed upon careful observation. 



As an example of variation in color and color pattern 

 notice the frontispiece, which shows thirty-five shells of 

 Ncrctina virginea variety minor selected from a thousand, 

 most of which were gathered by the author in the Salt Pond 



