36 ORTHOGENETIC EVOLUTION IN PIGEONS. 



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 varia- 

 tions in other directions by means of the elimination of those that exhibit them. (Evolu- 

 tion Theory, Vol. II, p. 123.) 



Carl von Nageli: 



Between the theory of selection and that of direct causation, there is, apparently, only 

 a little difference, since, according to the latter, the present condition of the organic world 

 would likewise result from individual variation and elimination. But these two processes 

 (selection and direct causation) differ fundamentally in their causal import. According 

 to Darwin, variation is the germinating factor, selection the directing and regulating 

 factor; according to my view, variation is at once both the germinating and the directing 

 factor. According to Darwin, selection is indispensable; without it there could be no pro- 

 gression, and organisms would remain in the same condition as at the beginning. In my 

 opinion, competition simply removes what is less capable of existence, but it is wholly 

 without influence in bringing to pass anything more perfect or better adapted. (Theorie 

 der Abstammungslehre, p. 285.) 



The fortuitous or directionless variation of individuals would be conceivable, if it were 

 conditioned by external influences (food, temperature, light, electricity, gravitation) ; for, 

 as these causes obviously can not be brought into any definite relation to the more or less 

 complex organization, they must effect sometimes a positive, sometimes a negative, step. 

 If, however, the causes of variation are internal, in the constitution of the substance, then 

 the matter stands otherwise. In this case the determinate organization of the substance 

 must exercise a restricting influence upon its own variation; and this influence, as develop- 

 ment begins at the lowest point, can only take effect in an upward direction. (Abstam- 

 mungslehre p. 12.) 



Individuals transmit to their offspring the tendency to be like themselves, but the 

 offspring are not perfectly like the parents. The tendency to variation must therefore 

 also be transmitted. A primordium, if all conditions are favorable, must be able to develop 

 ever farther in a series of generations, as a capital enlarges to which interest is added 

 annually; for each generation inherits from the preceding not only the possibility to realize 

 the capital, but also the possibility to add the interest. (Individuality in Nature, 1856.) 



Hugo de Vries: 



According to the theory of mutation, species have not arisen through gradual selection 

 continued for hundreds or thousands of years, but by steps, through sudden though small 

 transmutations. In contrast with variations, which are changes advancing in a linear 

 direction, the transformations to be called mutations diverge in new directions. They 

 take place, then, so far as experience goes, without definite direction, i.e., in various direc- 

 tions. (Die Mutationstheorie, Vol. I, p. 150.) 



