Cook: Are Species Uniform? 



287 



of evolution is largely discredited. 

 Changes are not confined to useful 

 characters. Many useless, non-adap- 

 tive differences are developed, and 

 useful features are specialized beyond 

 the point of utility. Even serious 

 defects are transmitted in latent form, 

 out of reach of natural selection, but 

 adding to the complexity of descent. 

 A tendency to spontaneous variation 

 must be assumed, even in cases where 

 characters are increased under artifi- 

 cial selection, as recognized by Castle. - 

 With a belief in normal uniformity 

 accepted, it is logical to argue that 

 changes in the characters of species 

 must require external agencies of the 

 environment or internal "mechanisms 

 of heredity," and such external or in- 

 ternal "causes of evolution" continue to 

 be sought by many investigators. A 

 different conception of evolution, re- 

 quiring no such assumptions of special 

 causes or mechanisms, becomes pos- 

 sible when the diversity that seems 

 everywhere to exist among the mem- 

 bers of species is reckoned as a normal 

 condition of heredity, and of evolu- 

 tionary progress.^ 



THE EVOLUTION OF SPECIES 



In order to think clearly and eftec- 

 tively about evolution we must form 

 practical conceptions of the changes 

 that go on in species, as representing 

 the evolutionary^ process. Species do 

 not differ as mutations or pure lines 

 differ, but in more general and less 

 definite ways, on account of individual 

 diversity. Evolution is to be thought 

 of not merely as change of individual 

 characters, or of characters of individ- 

 ual lines of descent, but as change in 

 characters of species, modifying event- 

 ually the whole network of descent so 

 that the members of a species can be 

 recognized as distinct from members of 

 the nearest related species. When this 

 stage has been reached, an evolution of 

 "new" species has been accomplished, 



by a process as gradual and indefinite 

 but no less real than the development 

 of a "new" language. 



As a new word does not make a new 

 language, so new characters do not 

 constitute new species, unless the 

 characters are preserved and estab- 

 lished in networks of descent. Many 

 writers go astray in assuming that 

 e\olution is merely originating new 

 characters, or is some special form of 

 character-origination, so that many 

 scientific works do not convey a clear 

 conception of the evolutionary process. 

 Professor Bateson recently has misled 

 Mr. Bryan into supposing that evolu- 

 tion is discredited in the scientific 

 world. The mistake has arisen because 

 both are looking for something that 

 probably never occurs, and should not 

 be expected to occur, a sudden trans- 

 formation of one species into another. 

 Darwin carefully considered and defi- 

 nitely rejected the idea of species origi- 

 nating abruptly, and this judgment 

 rests as more firmly established by the 

 efforts that ha^'e been made to dis- 

 place it. 



It is not in the nature of species as 

 networks of descent to originate by 

 definite, sudden changes of characters, 

 just as it is not in the nature of lan- 

 guages to be formed or changed sud- 

 denly. The Latin language was not 

 abruptly discarded or displaced in 

 Italy, Spain, or France, but local forms 

 of Latin developed gradually, and 

 eventually were recognized as distinct 

 languages. Mule-bred languages have 

 been elaborated and "new plant crea- 

 tions" have been produced and propa- 

 gated artificially, but such devices are 

 in contrast with normal development. 

 Diversity in words and forms of ex- 

 pression is universal in languages, like 

 diversity of characters in species, and 

 the relation of diversity to progress, in 

 furnishing the material of continued 

 evolutionary change, is a further anal- 

 ogy- 



2 Journal of the Washington Academy of Sciences, 7:387, June 19, 1917. 



3 Evolution not the Origin of Species, Popular Science Monthly, March 1904; The Nature of 

 Evolution, Science, N. S., Sept. 7, 1906; Methods and Causes of Evolution, Bui. 136, Bureau of 

 Plant Industrv, October 1908; Pure Strains as Artifacts of Breeding, The American Naturalist, 

 April 1909. 



