42 Darwin, and after Darwin. 



form or method of the process. But where the question 

 is as to whether or not the theory is true, it becomes 

 a mere begging of this question to take refuge in the 

 argument from ignorance, or to represent in effect 

 that there is no question to be discussed. And if, 

 when the form or method is investigated, it be found 

 everywhere charged with evidence in favour of the 

 theory of descent, the case becomes the same as that 

 of a supposed revelation, which has been discredited 

 by finding that all available evidence points to a 

 natural growth. In short, the argument from ignor- 

 ance is in any case available only as a negative foil 

 against destructive criticism : in no case has it any 

 positive value, or value of a constructive kind. There- 

 fore, if a theory on any subject is destitute of positive 

 evidence, while some alternative theory is in possession 

 of such evidence, the argument from ignorance can be 

 of no logical use to the former, even though it may be 

 of such use to the latter. For it is only the possession 

 of positive evidence which can furnish a logical justifi- 

 cation of the argument from ignorance : in the absence 

 of such evidence, even the negative value of the argu- 

 ment disappears, and it then implies nothing more 

 than the gratuitous assumption of a theory. 



I will now sum up the various considerations which 

 have occupied us during the present chapter. 



First of all we must take note that the classification 

 of plants and animals in groups subordinate to groups 

 is not merely arbitrary, or undertaken only for a 

 matter of convenience and nomenclature such, for 

 instance, as the classification of stars in constellations. 

 On the contrary, the classification of a naturalist 



