BEAUTY, HOW ACQUIRED. 195 



the latter fortunately is most rare, is all the same to the 

 inexorable principles of natural selection. If we admire 

 the several ingenious contrivances by which orchids and 

 many other plants are fertilized through insect agency, can 

 we consider as equally perfect the elaboration of dense 

 clouds of pollen by our fir-trees, so that a few granules 

 may be wafted by chance on to the ovules? 



SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CON- 

 DITIONS OF EXISTENCE EMBRACED BY THE THEORY OF 

 NATURAL SELECTION. 



We have in this chapter discussed some of the difficulties 

 and objections which may be urged against the theory. 

 Many of them are serious; but I think that in the discussion 

 light has been thrown on several facts, which on the belief 

 of independent acts of creation are utterly obsure. We 

 have seen that species at any one period are not indefinitely 

 variable, and are not linked together by a multitude of 

 intermediate gradations, partly because the process of 

 natural selection is always very slow, and at any one time 

 acts only on a few forms; and partly because the very pro- 

 cess of natural selection implies the continual supplanting 

 and extinction of preceding and intermediate gradations. 

 Closely allied sj^ecies, now living on a continuous area, 

 must often have been formed when the area was not con- 

 tinuous, and when the conditions of life did not insensibly 

 graduate away from one part to another. When two vari- 

 eties are formed in two districts of a continuous area, an 

 intermediate variety will often be formed, fitted for an 

 intermediate zone; but from reasons assigned, the inter- 

 mediate variety will usually exist in lesser numbers than 

 the two forms which it connects; consequently the two 

 latter, during the course of further modification, from ex- 

 isting in greater numbers, will have a great advantage over 

 the less numerous intermediate variety, and will thus gen- 

 erally succeed in supplanting and exterminating it. 



We have seen in this chapter how cautious we should be 

 in concluding that the most different habits of life could 

 not graduate into each other; that a bat, for instance, 

 could not have been formed by natural selection from an 

 animal which at first only glided through the air. 



