206 
DIFFICULTIES ON THEOEY. 
Chap. VI. 
the larger country there will have existed more indi¬ 
viduals, and more diversified forms, and the competition 
will have been severer, and thus the standard of perfec¬ 
tion will have been rendered higher. Natural selection 
will not necessarily produce absolute perfection; nor, as 
far as we can judge by our limited faculties, can absolute 
perfection be everywhere found. 
On the theory of natural selection we can clearly 
understand the full meaning of that old canon in natural 
history, ‘^Natura non facit saltum.” This canon, if 
we look only to the present inhabitants of the world, is 
not strictly correct, but if we include all those of past 
times, it must by my theory be strictly true. 
It is generally acknowledged that all organic beings 
have been formed on two great laws—Unity of Type, 
and the Conditions of Existence. By unity of type is 
meant that fundamental agreement in structure, which 
we see in organic beings of the same class, and which is 
quite independent of their habits of life. On my theory, 
unity of type is explained by unity of descent. The 
expression of conditions of existence, so often insisted on 
by the illustrious Cuvier, is fully embraced by the prin¬ 
ciple of natural selection. For natural selection acts by 
either now adapting the varying parts of each being to 
its organic and inorganic conditions of life ; or by having 
adapted them during long-past periods of time : the 
adaptations being aided in some cases by use and dis¬ 
use, being slightly affected by the direct action of the 
external conditions of life, and being in all cases sub¬ 
jected to the several laws of growth. Hence, in fact, 
the law of the Conditions of Existence is the higher 
law; as it includes, through the inlieritance of former 
adaptations, that of Unity of Type. 
