ORIGIN OF LIFE. 55 
Before we can be at liberty to conclude that some- 
thing is universally true because we have never known 
an instance to the contrary, we must -have reason to 
believe that if there were in nature any instances to 
the contrary, we should have known of them.” Now 
it was only by an utter inattention to this latter all- 
important requirement that the “ past experience of 
mankind” could ever have appeared to warrant the 
induction omue vivum ex vivo. 
As Mr. Mill pointed out,* the proposition, “all 
swans are white,” must have appeared to Europeans, 
not many years ago, an “unequivocal instance of 
uniformity in the course of nature.” Subsequent 
experience has shown that they were mistaken, 
although they and all their predecessors through 
many centuries had observed nothing to contradict 
this proposition. “The uniform experience therefore 
of the inhabitants of the known world, agreeing in 
a common result, without one known instance of 
deviation from that result, is not always sufficient 
to establish a general conclusion.” 
(6). The exposure of the untruth of certain old 
and crude doctrines concerning ‘spontaneous genera- 
tion,’ many of which date from the earliest times ; and 
the fact that the belief in this mode of generation has 
been successively driven, with increasing knowledge, 
* Loc. cit. vol. i., pp. 348, 351- 
