32 Mutability and Individual Variation. 



his theory. Here, as everywhere, Darwin advanced 

 with the utmost caution. 



Our problem then is this : In the formation of new 

 species, does natural selection choose the extreme vari- 

 ants of the ordinary individual variation, or does it choose 

 occasional Mutations. In any large community there is 

 always an abundant supply of extreme variants. More- 

 over the struggle for existence does not preserve the 

 single absolutely perfect ones only, but groups of the 

 best, since it simply eliminates the least perfectly adapted. 

 There is, so to speak, always plenty of material for se- 

 lection in every species, and in every character. But in- 

 dividual variability is, as far as our experience goes, by 

 no means unlimited ; its limits are not indeed precise, but 

 thev fall well within the rano-e of Ouetelet's Law. 



Single variations are chance phenomena into whose 

 essential nature Ave have as yet no insight. We know 

 that they occur and that they occur seldom ; but not too 

 seldom. As to how they come about scarcely anything 

 is known, but it is generally assumed that they appear 

 suddenly,^ and they are consequently termed sports. They 

 suddenly change a species into a new form ; or, from a 

 variety, they make a new one absolutely different. Fre- 

 quently they concern only a single character and then 

 usually consist in the loss or latency of a character al- 

 ready present, e. g., white flowers, absence of thorns 

 {Datura incrmis, Fig. 5), hairs, runners (e.g., Fragaria 

 alpina, Figs. 6 and 7, pp. 33 and 34),^ seeds, branching, 



^ By far the majority of observations that have been adduced as 

 instances come under the heading of hybridization. 



^The Gaillon strawberries (Fig. 7) which are distinguished 

 from the ordinary monthly strawberries (Fragaria alpina, Fig. o; 

 solely by the absence of suckers and the correspondingly greater 

 branching of the rosettes are often cultivated for the very reason of 



