Darwin- Wallace Celebration. 97 



of propagating their new and slightly different structure ; 

 and the modification may be slowly increased by the accumu- 

 lative action of natural selection to any profitable extent. 

 The variety thus formed will either coexist with, or, more 

 commonly, will exterminate its parent form. An organic 

 being, like the woodpecker or misseltoe, may thus come to 

 be adapted to a score of contingences natural selection 

 accumulating those slight variations in all parts of its 

 structure, which are in any way useful to it during any part 

 of its life. 



5. Multiform difficulties will occur to every one, with 

 respect to this theory. Many can, I think, be satisfactorily 

 answered. Natura non facit saltuin answers some of the 

 most obvious. The slowness of the change, and only a very 

 few individuals undergoing change at any one time, answers 

 others. The extreme imperfection of our geological records 

 answers others. 



6. Another principle, which may be called the principle 

 of divergence, plays, I believe, an important part in the 

 origin of species. The same spot will support more life if 

 occupied by very diverse forms. We see this in the many 

 generic forms in a square yard of turf, and in the plants or 

 insects on any little uniform islet, belonging almost invariably 

 to as many genera and families as species. We can under- 

 stand the meaning of this fact amongst the higher animals, 

 whose habits we understand. We know that it has been 

 experimentally shown that a plot of land will yield a greater 

 weight if sown with several species and genera of grasses, 

 than if sown with only two or three species. Now, every 

 organic being, by propagating so rapidly, may be said to be 

 striving its utmost to increase in numbers. So it will be 

 with the offspring of any species after it has become 

 diversified into varieties, or subspecies, or true species. And 

 it follows, I think, from the foregoing facts, that the varying 

 offspring of each species will try (only few will succeed) to 

 seize on as many and as diverse places in the economy of 

 nature as possible. Each new variety or species, when 

 formed, will generally take the place of, and thus extermi- 

 nate its less well-fitted parent. This I believe to be the 

 origin of the classification and affinities of organic beings at 

 all times ; for organic beings always seem to branch and 

 sub-branch like the limbs of a tree from a common trunk, 

 the flourishing and diverging twigs destroying the less 

 vigorous the dead and lost branches rudely representing 

 extinct enera and families. 



