THE NATURE OF SPECIES 



impassable. Such units were once supposed to exist, and they 

 were called species, a word meaning a kind. The name 

 species was given under the impression that, though each 

 kind so designated might vary to some extent, the variations 

 were restricted within limits that could not be transgressed. 

 The differences between members of the same species were 

 regarded as individual variations. Slight differences between 

 plants and animals of the same species were recognized by 

 careful observers, and the abrupt changes seen in sports and 

 monstrosities attracted the attention of the curious. Many 

 early authors placed no limit on the extent to which such 

 variations might occur. Bacon observed a fern growing out 

 of a willow and, instead of explaining it as a natural graft 

 due to a windblown spore caught in a crack, regarded it as 

 an offshoot due to some injury or some special influence. 

 He also suggested that the stump of a felled beech might put 

 forth birch, it being "a tree of a smaller kind which needeth 

 less nourishment." Thus, according to Bacon, a beech might 

 be developed into a birch by an unfavourable environment. 



The belief in the fixity of species arose in the generation 

 after Bacon. Herbert Spencer ^ attributed it to a literal 

 acceptance of the Mosaic account of Creation, and it has 

 often been credited to Milton, who relates, in Paradise Lost,^ 

 how, on the sixth day, in accordance with the Divine com- 

 mand. 



The earth obeyed, and, straight, 

 Opening her fertile womb, teemed at a birth, 

 Innumerous living creatures, perfect forms, 

 Limbed and full grown. 



But, as Professor Poulton has remarked, the belief in the 

 sudden appearance of animals in their present forms was 



^ In 1852, reprinted in Essays, vol. I, p. 583, 1868. 

 ^ Book VII, lines 387-500. 



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