194 
DIFFICULTIES ON THEOKY. 
Chap. YI. 
the very same invention, so natural selection, working 
for the good of each being and taking advantage of 
analogous variations, has sometimes modified in very 
nearly the same manner two parts in two organic beings, 
which beings owe but little of their structure in common 
to inheritance from the same ancestor. 
Although in many cases it is most difficult to con¬ 
jecture by what transitions organs could have arrived at 
their present state; yet, considering that the proportion 
of living and known forms to the extinct and unknown 
is very small, I have been astonished how rarely an 
organ can be named, towards which no transitional 
grade is known to lead. The truth of this remark is 
indeed shown by that old but somewhat exaggerated 
canon in natural history of ^^Natura non facit saltum.” 
We meet with this admission in the writings of almost 
every experienced naturalist; or, as Milne Edwards has 
well expressed it, Nature is prodigal in variety, but 
niggard in innovation. Why, on the theory of Creation, 
should this be so ? Why should all the parts and organs 
of many independent beings, each supposed to have been 
separately created for its proper place in nature, be so 
commonly linked together by graduated steps ? Why 
should not Nature have taken a leap from structure to 
structure ? On the theory of natural selection, we can 
clearly understand why she should not; for natural se¬ 
lection can act only by taking advantage of slight suc¬ 
cessive variations; she can never take a leap, but must 
advance by the shortest and slowest steps. 
Organs of little apparent importance ,—As natural 
selection acts by life and death,—by the preservation of 
individuals with any favourable variation, and by the 
destruction of those with any unfavourable deviation of 
structure,—I have sometimes felt much difficulty in 
