January 15, 1915] 



SCIENCE 



95 



for example, are less and less like modem 

 horses the more remote in geological time are 

 the deposits in which their bones are found. 

 But students of evolution differ in their ideas 

 as to how gradual the progress of evolution has 

 been and is, for no one supposes the process 

 ended. 



Within my view stands the sloping bank of 

 a reservoir, which most visitors ascend by a 

 flight of granite steps; but children often go 

 up the grassy bank wherever they happen to 

 encounter it. Either method takes one to the 

 top, the gradual or the stepwise mode of 

 ascent. 



Evolution was thought by Darwin to occur 

 in two ways comparable with these, the grad- 

 ual and the stepvidse. From Darwin's vTritings 

 it would seem that he regarded the gradual 

 as the more common and important method 

 of evolutionary change among organisms, but 

 it is clear that he recognized stepwise or 

 " sport " variation as of considerable value, 

 particularly in the production of new vari- 

 eties under domestication. 



But not many years after Darwin's death a 

 question arose in the minds of certain thought- 

 ful naturalists as to whether Darwin had 

 rightly estimated the relative importance of 

 these two methods of evolution. Galton, 

 Bateson and DeVries have laid increasing 

 emphasis on sport variations or " mutations," 

 until these have come to be regarded by many 

 as of overshadowing importance in evolution. 

 The full-fledged mutation theory^ maintains 

 that evolution occurs hy steps alone, that is 

 that new species arise from old ones by single 

 discontinuous steps, never by gradual unin- 

 terrupted change. This theory has been the 

 guiding principle in evolutionary study in 

 recent years. Its basic idea is that natural 

 species are invariably discontinuous and that 

 intergrades between them do not occur except 

 possibly as the result of sporadic hybridization, 



2 The term ' ' mutation theory ' ' is here used in 

 its widest sense, including not merely the ideas of 

 De Vries concerning evolution among the evening 

 primroses, but the general idea of discontinuity 

 in the origin of species previously outlined by 

 Galton and Bateson. 



such intermediate forms being unstable and 

 so without significance. The attempt by Bate- 

 son^ to classify the variations which occur 

 within species led him to the conclusion that 

 only such variations as are discontinuous in 

 nature can have species-forming value, since 

 they alone are not " swamped by crossing." 

 This idea has been supported by the observa- 

 tion that among species regularly dimorphic 

 or polymorphic, the several forms which re- 

 main distinct, notwithstanding constant inter- 

 crossing, are Mendelian alternatives, conform- 

 ing with the laws of dominance and segrega- 

 tion. Many of the striking variations in color 

 and form which occur among domesticated 

 animals and plants follow these same laws so 

 that their rediscovery and verification in 1900 

 was rightly regarded as strong presumptive 

 proof of discontinuity in evolution. At about 

 the same time DeVries brought together in 

 his book entitled " The Mutation-Theory " a 

 large amount of evidence favoring the idea of 

 discontinuity in evolution most important of 

 which was the repeatedly verified polsrmorph- 

 ism of the seedlings produced by Lamarck's 

 evening-primrose. 



Mendelian segregation, however, does not at 

 present offer a sufficient explanation of muta- 

 tion in the evening-primroses so that provi- 

 sionally we are forced to conclude with Gates* 

 that mutation and segregation following 

 hybridization are probably distinct phenomena. 

 It also remains doubtful whether the phe- 

 nomena observed among evening-primroses 

 occur at all commonly among other plants or 

 among animals. The so-called " mutations " 

 which Morgan has observed in the fly Droso- 

 phila are certainly not of this order, but are 

 clearly due to Mendelian factorial variation. 

 Many with Bateson think that Mendelism 

 affords a basis for the explanation of all evo- 

 lution and confidently expect the evening- 

 primroses sooner or later to be shown con- 

 formable with its fundamental ideas. In the 



3 "Materials for the Study of Variation," 1894. 



* Gates, K. E., ' ' Breeding Experiments which 

 Show that Hybridization and Mutation are Inde- 

 pendent Phenomena," Zeits. f. ind. Abst. u. 

 Vererlungslehre, 11, pp. 209-279, 1914. 



