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more normal, more vigorous, and more fertile, than their parents. 

 Why, then, are they called hybrids? Because we have been 

 led astray by the theory of normally uniform and stationary 

 species, in which it was made to appear that anything which 

 interfered with identity of form and structure was essentially 

 unnatural, like a cross between members of species which do 

 not normally breed together, and which produce, when so bred, 

 abnormal progeny. There are many groups in nature which 

 are reckoned as species, but which are no farther apart than 

 some of the varieties of cultivated plants, and which can breed 

 together without difficulty or abnormality. For systematic 

 purposes it is desirable to recognize each separate natural 

 group of organisms as a species, and this can also be justified 

 from evolutionary standpoints, for segregated groups are able 

 to make evolutionary progress on distinct lines, and eventually 

 to become different from other groups of common origin. 



It often happens, however, that evolutionary progress is not 

 consistent in the vegetative and reproductive parts of the organ- 

 isms. Species which appear very distinct externally may, when 

 brought together, breed freely and normally, while others whose 

 bodily differences are difficult to detect may refuse to mingle 

 or may produce only sterile or otherwise abnormal hybrids. 

 While it is thus difficult or, it may be, impossible, to draw an 

 absolute line of definition, or to restore the old distinction be- 

 tween hybrids and crosses, this does not justify us in ignoring 

 the very wide and very practical differences between the ex- 

 treme conditions of this series of phenomena. 



ANALOGIES OF HYBRIDS AND MUTATIONS. 



The phenomena which have the nearest and most genuine 

 relations with hybrids are not crosses, but mutations. Hybrids 

 and mutations can both be described in the same words, as 

 aberrations from normal heredity. Both are due to the same 

 cause, inadequate fertilization, which unbalances the organic 

 equilibrium and gives rise to abrupt variation, usually in many 

 directions at once. Mutations and hybrids show also a general 

 deficiency of fertility. This is carried, very often, to the ex- 

 treme of complete sterility, though there may be present at the 



