OTHER THEORIES OF SPECIES-FORMING. 3 6 7 



any important way will feel convinced that they are not capable 

 of forming intergrades and hence could not have arisen gradu- 

 ally." 



24 C. B. Davenport ("Evolution Without Mutation," Jour, of 

 Exper. Zool, Vol. II, pp. I37-M3, 1905), in a recent short paper, 

 adduces facts concerning the variation and evolution 

 examplesflrfspe- of Pectens, which lead him to conclude that the races 

 oies-origin by of Pecten inhabiting different geographical regions 

 slight continu- are connected so plainly by integrating variations 

 ous change, ^at there can be no question of mutations in con- 

 nection with their origin. They must have arisen through evolu- 

 tion by trivial variation. Davenport concludes his paper with the 

 following summary: "The process of evolution has taken place by 

 various methods and not always in the same way. It is not more 

 justifiable to maintain that all evolution is by mutation than that 

 evolution has always proceeded by slow stages. The best evidence 

 for slow evolution is found in wide-ranging species which, while 

 differing greatly at the limits of their range, exhibit all gradations in 

 intermediate localities (Melospiza, Pecten} ; also in fossil series 

 (Pecten eboreus and P. irradians} where the change from one hori- 

 zon to the next is of a quantitative order. Thus evolution may take 

 place without mutation." 



Naturalists whose special field of study is systematic and fau- 

 nistic rather than morphologic or experimental, seem to be slow to 

 Memam's nn< ^ much good in the mutations theory. Indeed, 

 criticism of the some of them seem to be quick to find much that is 

 mutations theory, \\\ m j^ For example, Merriam and Allen, deans of 

 American faunistic students of birds and mammals, are both strong 

 antagonists of the mutations theory. Merriam, in an address as 

 chairman of the zoological section at the 1905 meeting of the Ameri- 

 can Association for the Advancement of Science ("Is Mutation the 

 Factor in the Evolution of the Higher Vertebrates ?" Science, N. S., 

 Vol. XXIII, pp. 241-257, 1906), shows by the use of illustrations- 

 drawn from the distribution and taxonomy of American chipmunks 

 and ground squirrels that the mutations theory cannot by any 

 means explain all species-forming. "My argument," says Merriam, 

 "is not that species of plants may not in rare cases arise in perpetu- 

 ation of sport characters, as de Vries believes they do, but admitting 

 this, my contention is that the overwhelming majority of plants,, 

 and so far as known, all animals, originate in the generally recog- 

 nised way by the gradual development of minute variations. The 

 theory of origin of species by mutation, therefore, far from being 

 a great principle in biology, as some seem to believe, appears to be 

 one of a hundred minor factors to be considered in rare cases as a. 



