THE VITAL FABRIC OF DESCENT 3II 



ascribed to mutations. The fact that one member of a group of 

 inbred individuals has mutated, is accepted as an excellent rea- 

 son for believing that others are ready for the same step, thus 

 explaining at once the relative prepotency of a mutation under 

 continued inbreeding, and its " reversion " in the presence of 

 the wild stock or of a more symbasic breed. On the other hand, 

 the crossing of two mutations of distinct ancestry, even though 

 of closely similar form, constitutes a decrease of inbreeding, 

 and carries with it a possibility of restoration to the normal type. 

 Darwin found that crosses of unrelated white pigeons " reverted" 

 to the blue plumage of the wild type, but he did not hold that 

 such precarious, pathological variations are factors in the evo- 

 lution of species in nature. Symptoms of disease have often 

 helped, however, to understandings of healthy functions. 



Mutations are abnormal manifestations of the normal phe- 

 nomenon of variation or diversity inside the species. The pre- 

 potency of mutations when bred with their own inbred relatives 

 corresponds to the prepotency of normal variations. The 

 " reversion" or negative prepotency of a mutation in the pres- 

 ence of a more widely symbasic stock does not prove that new 

 species originate in nature by the segregation of mutations ; it 

 simpl}^ increases the improbability of a general theory of evolu- 

 tion built on the narrow basis of the mutations of domesticated 

 plants and animals. 



The rejection of the hypothesis of the origin of species through 

 mutation does not make it necessary to disregard any of the facts 

 which have been collected to support it. The objection is not 

 to the data, but to the generalization, and to the use of a stand- 

 point which can be maintained only while other equally perti- 

 nent facts are disregarded. 



In his report of experiments on " Color Inheritance in Mice "^ 

 Professor Davenport notes that albino mice of mixed parentage 

 were found to be more prepotent, or less completely recessive 

 than those of pure descent. Instead of more gray progeny 

 as an inheritance from the gray parent, they gave a larger pro- 

 portion of white offspring, a result as directly in accord with the 

 kinetic theory as it is at variance with the current mechanical 



1 Science, N. S., 19: no, January 15, 1904. 



