42 Darwin, and after Darwin. 
form or method of the process. But where the question 
is as to whether or not the theory zs 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 
