120 THE WORLD OF LIFE 



greater range of variation, and an ever-present abundance of 

 variations of all the parts and organs of the species. 



In my Darwinism (chapter iii.) I have given sixteen dia- 

 grams of variation, showing that it ocenrs to an approximately 

 equal extent in mammals and reptiles as well as in birds, and 

 in a large number of their parts and external organs; while 

 many examples of variation occur among the lower animals, 

 especially insects, and also to an amazing extent among plants. 

 During the last twenty years an enormous amount of work 

 has been done in the investigation of variation in all its phases 

 and complexities, and an excellent accoimt of these has been 

 given by Dr. H. M. Yernon in his Variation in Animals and 

 Plants, 1903 (International Scientific Series), to which my 

 readers are referred for fuller information, but a few of his 

 conclusions may be here given. He says : 



" Every organism varies in respect of all its characters, what- 

 ever be their nature. The amount of this variation differs greatly, 

 but it is always present in a greater or less degree." 



And again, referring to a diagram showing the variations of 

 a squirrel, he says: 



" Variation of a similar nature — though of a varying degree — 

 is present in all organisms, to whatever class of the animal or 

 vegetable kingdom they belong." 



Eef erring to the diagram of human stature at p. 116, it is 

 found that about half the whole number measured vary a little 

 more than 2 inches above or below the mean, or a little more 

 than 3 per cent of the mean height. This is termed the per- 

 centage of mean error, and Mr. Vernon gives us an interesting 

 table of the same percentage for different parts of the body 

 derived from very larg-e numbers of measurements of different 

 races of men. It is as follows : — 



