194 DIFFICULTIES ON THEOEY. Chap. VI. 



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 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 an organ could have arrived 

 at its 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 canon in natural history of 

 " Natura non facit saltum." We meet with this admis- 

 sion in the writings of almost every experienced natu- 

 ralist; 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 inde- 

 pendent beings, each supposed to have been separately 

 created for its proper place in nature, be so invariably 

 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 selection 

 can act only by taking advantage of slight successive 

 variations ; she can never take a leap, but must ad- 

 vance 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 



