PROBLEM OF ORIGIN OF SPECIES 57 



existing among five hundred or more species of pigeons, and even to 

 trace affinities farther back in the bird world. 



The orthogenetic process is the primary and fundamental one. 

 In its course we find unlimited opportunities for the play of natural 

 selection, escape the great difficulty of incipient stages, and readily 

 understand why we find so many conditions arising and persisting 

 without any direct help of selection. 



Charles Darwin. 



"As natural selection acts solely by accumulating slight, successive, favor- 

 able variations, it can produce no great or sudden modification." Origin of 

 Species, ch. xiv, p. 421. 



" Slight individual differences, however, suffice for the work, and are probably 

 the sole differences which are effective in the production of new species." 

 Animals and Plants, vol. n, ch. xx, p. 233. 



" As modern geology has almost banished such views as the excavation of a 

 great valley by a single diluvial wave, so will natural selection, if it be a true 

 principle, banish the belief of the continued creation of new organic beings, or of 

 any great and sudden modification in their structure." Origin of Species, ch. iv, 

 p. 98. 



August Weismann. 



"The simultaneous modification of numerous cofunctioning parts, in essen- 

 tially different ways, yet in harmonious functional relations, points conclu- 

 sively to the fact that something is still wanting to the selection of Darwin and 

 Wallace." Germinal Selection, p. 22. 



"We know of only one natural principle of explanation for adaptation, that 

 of selection." Ibid., p. 61. 



"The three principal stages of selection; that of personal selection, as held 

 by Darwin and Wallace; that of histonal selection, as upheld by Wilhelm Roux 

 in the form of a 'Struggle of the Parts;' and finally, that of germinal selection, 

 the existence of which I have endeavored to establish these are the factors 

 that cooperate to maintain the forms of life constantly capable of life." Ibid., 

 p. 60. 



"The harmony of the direction of variation with the requirements of the 

 conditions of life is the riddle to be solved. The degree of the adaptation which 

 a part possesses itself determines the direction of variation of that part." Ibid., 

 p. 54. 



" When a determinant has assumed a certain variation-direction it will follow it 

 up of itself, and selection can do nothing more than secure it a free course by setting 

 aside variations in other directions by means of the elimination of those that exhibit 

 them." Evolution Theory, vol. n, p. 123. 



Carl von Nageli. 



" Between the theory of selection and that of direct causation, there is, appar- 

 ently, only a little difference, since, according to the latter, the present condi- 

 tion of the organic world would likewise result from individual variation and 

 elimination. But these two processes [selection and direct causation] differ fun- 

 damentally in their causal import. According to Darwin, variation is the germi- 

 nating factor, selection the directing and regulating factor; according to my 

 view, variation is at once both the germinating and the directing factor. Accord- 

 ing to Darwin, selection is indispensable; without it there could be no progres- 

 sion, and organisms would remain in the same condition as at the beginning. 



