November 2, 1906.] 



SCIENCE. 



557 



individual variations ' would not have been 

 asked if he had bred sports and observed their 

 resistance to blending and reversion. In 

 poultry, tailless birds bred to tailed birds pro- 

 duce in the second generation and later a 

 large proportion of vs'holly tailless offspring. 

 Crest, frizzled feathers, tendency to produce 

 pigment in the connective tissue, dominate 

 over the normal conditions. When birds with 

 a down-like modification of the adult plumage 

 are bred together all of the offspring have that 

 peculiarity. Even the polydactile condition 

 does not blend with the normal. After these 

 facts what becomes of the ' opinion ' that has 

 been bandied about for over a generation and 

 is resuscitated by Merriam that sports are 

 lost by swamping? Regrettable is Merriam's 

 innuendo directed toward zoologists who have 

 been trained in analytical methods involving 

 "the use of the microscope. I am not con- 

 vinced that analytical training in the labora- 

 tory is a less adequate training for tackling 

 the species problem than setting traps and 

 shooting and skinning mammals and birds in 

 the field. What the problem demands is an 

 analysis of species into their constituent 

 characteristics, a study of the behavior of 

 these characteristics in the field and the labo- 

 ratory under controlled environmental condi- 

 tions and a study of their inheritance. 



Certain special cases are cited at some 

 length by Merriam as disproving the mutation 

 theory. The number might have been greatly 

 increased. In general it may be said the 

 widely ranging species of small mammals and 

 non-migratory birds in North America exhibit 

 remarkable parallel changes in coloration, 

 gaining a darker and brighter pigmentation 

 as one passes from the dry plains or the in- 

 terior deserts to the moister Pacific coast from 

 northern California to Alaska. It is hardly 

 conceivable that mutations of exactly the same 

 sort should affect so many species in the 

 same way. It seems more reasonable to as- 

 cribe these changes to climatic conditions. 

 Whether these changes are permanent enough 

 to warrant calling them specific character- 

 istics is uncertain. Mr. Chapman tells me 

 that there is evidence that in the case of 

 <!ertain species originating in the interior, 



one section spread to the southern deserts, 

 where it became pale, while another section 

 spread northward, where it was rendered dark, 

 and that these two sections have subsequently 

 approached each other on the Pacific coast, 

 where they are strikingly dissimilar, although 

 near together. Such a history, which de- 

 serves working out in detail, would indicate a 

 persistence of the climatic modifications. I, 

 for one, am quite satisfied that geographic 

 variation, determined directly or indirectly by 

 climate, is not of the order of mutation and 

 may involve a permanent modification of the 

 germ-plasm. The variation is often sufficient 

 to justify assignment of the extremes to dis- 

 tinct species whenever intergrades are miss- 

 ing. These geographic variations are, how- 

 ever, it must be confessed, largely of a quan- 

 titative rather than of a qualitative sort; i. e., 

 usually new characters are not involved, but 

 only a modification of characters, as, e. g., in 

 the case of mammals an increase of the black 

 and red pigment. It may well be that species 

 founded on quantitative differences follow a 

 different method of evolution than those 

 founded on qualitative ones. I, for one, while 

 an adherent of the mutation theory, still 

 maintain the view expressed by me in 1904,'' 

 when I used some of the same kind of evi- 

 dence employed by Merriam. This view is 

 that the process of evolution is complex 

 enough to admit of many ' factors ' and evolu- 

 tion is not always by mutation. 



The real argument for discontinuity in 

 evolution is the occurrence of characteristics 

 in nature that are discontinuous and which 

 never show intergrades. The mere fact of 

 discontinuity between species of the same 

 genus is not sufficient to prove that they have 

 arisen by mutation. It must be shown that 

 the differential characters are in essence dis- 

 continuous. The practical way to get at the 

 true nature of characteristics, whether con- 

 tinuous or discontinuous, is by their behavior 

 in inheritance. If, in cross-breeding, a char- 

 acter tends to blend with the dissimilar char- 

 acter of its consort it must be concluded that 

 the character can be fractionized and that 



- ' Evolution without Mutation,' Journal of Ex- 

 perimental Zoology, II., pp. 137-143. 



