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4. The evolution theory presents a most interesting and 
instructive case for the application of this logic. Its main 
points are: that what we supposed to be distinct genera of 
animated beings did not originate in the creation of first 
progenitors, from whom all the subsequent individuals de- 
scended by a generation which transmitted, by propagation, 
precisely the properties essential to the genus; but that 
higher genera were slowly evolved from lower; that the 
causes of the differentiations wherein the more developed 
individuals differ from their less developed progenitors, are to 
be found in three unintelligent physical influences,—heredity, 
the influence of the environment on the being’s powers, and 
the survival of the fittest. The observed facts from which 
this hypothesis claims to derive its induction may be grouped 
under these general statements: that in fact the known genera 
of animated beings form a continuous ascending scale, from 
the most rudimental up to man, the most highly organised ; 
thus suggesting the ascent of organisation along this ladder, 
from a lower stage to a higher; that a multitude of organs 
and limbs are actually seen to grow from their infantile to 
their adult states, under the interaction of their environment 
and the instinctive animal exertions of them; that the con- 
ditions of animal existence are, in the general, such that the 
individuals possessing most of the natural vigour, qualifying 
them to reproduce a strong or a developed progeny, are most 
likely to survive, while the less qualified perish; and that 
observed facts in the breeding of animals present cases in 
which the rule does not hold that ‘‘ Like produces only its 
like,” but often it produces the slightly unlike, differing from 
itself by aslight shade of improvement or deterioration. These 
facts, the theory claims, when a very long time is allowed for 
the slow and irregular, but in the main progressive, action of 
the forces they disclose, prove that all animated genera can be 
accounted for as the ultimate progeny of the most rudimental 
protozoon. 
The task in hand here is not to give a full refutation of this 
theory, but to criticise it in the light of the logical principles 
established, simply in order to see whether it is an induction. 
It appears at once that it has no claim to come under the 
head of either method of induction, not even of the loosest, 
the method of agreement. Indeed, it cannot be said to have 
a single instance (much less an agreeing multitude) in the 
proper sense of inductive instances. ‘To resort for simplifica- 
tion to our notation, let A stand for the aggregate of supposed 
evolutional agencies, which are the combined cause; let X 
stand for the effect, a new genus. There has not been pre- 
