104 Evolution and Adaptation 



The facts of observation show, that when a new variety 

 appears its descendants are more likely, on the average, to 

 produce proportionately more individuals that show the same 

 variation, and some even that may go still farther in the same 

 direction. If these latter are chosen to be the parents of the 

 next generation, then once more the offspring may show the 

 same advance ; but little by little the advance slows down, 

 until before very long it may cease altogether. Unless, then, a 

 new kind of variation appears, or a new standard of variation 

 develops of a different kind, the result of selection of fluctu- 

 ating variations has reached its limit. Our experience seems, 

 therefore, to teach us that selection of fluctuating variations 

 leads us to only a certain point, and then stops in this direc- 

 tion. We get no evidence from the facts in favor of the 

 view that the process, if carried on for a long time, could 

 ever produce such great changes, or the kind of changes, as 

 those seen in wild animals and plants. 



Variation and Competition in Nature 



Darwin rests his theory on the small individual variations 

 which occur in nature, as the following quotation shows : — 



" It may be doubted whether sudden and considerable 

 deviations of structure such as we occasionally see in our 

 domestic productions, more especially with plants, are ever 

 permanently propagated in a state of nature. Almost every 

 part of every organic being is so beautifully related to its 

 complex conditions of life that it seems as improbable that 

 any part should have been suddenly produced perfect, as that 

 a complex machine should have been invented by man in a 

 perfect state. Under domestication monstrosities sometimes 

 occur which resemble normal structures in widely different 

 animals. Thus pigs have occasionally been born with a sort 

 of proboscis, and if any wild species of the same genus had 

 naturally possessed a proboscis, it might have been argued 



