74 DARWINISM chap. 



common species they are sure to be found ; that they are 

 everywhere of considerable amount, often reaching 20 per 

 cent of the size of the part implicated ; and that they are to 

 a great extent independent of each other, and thus afford 

 almost any combination of variations that may be needed. 



It must be particularly noticed that the whole series of 

 variation-diagrams here given (except the three which illustrate 

 the number of varying individuals) in every case represent the 

 actual amount of the variation, not on any reduced or enlarged 

 scale, but as it were life-size. Whatever number of inches or 

 decimals of an inch the species varies in any of its parts is 

 marked on the diagrams, so that with the help of an ordinary 

 divided rule or a pair of compasses the variation of the 

 different parts can be ascertained and compared just as if the 

 specimens themselves were before the reader, but with much 

 greater ease. 



In my lectures on the Darwinian theory in America and 

 in this country I used diagrams constructed on a different 

 plan, equally illustrating the large amount of independent 

 variability, but less simple and less intelligible. The present 

 method is a modification of that used by Mr. Francis Galton 

 in his researches on the theory of variability, the upper line 

 (showing the variability of the body) in Diagrams 4, 5, 6, and 

 13, being laid down on the method he has used in his experi- 

 ments with sweet-peas and in pedigree moth-breeding. 1 I be- 

 lieve, after much consideration, and many tedious experiments 

 in diagram-making, that no better method can be adopted for 

 bringing before the eye, both the amount and the peculiar 

 features of individual variability. 



Variations of the Habits of Animals. 



Closely connected with those variations of internal and 

 external structure which have been already described, are the 

 changes of habits which often occur in certain individuals or 

 in whole species, since these must necessarily depend upon some 

 corresponding change in the brain or in other parts of the 

 organism; and as these changes are of great importance in 

 relation to the theory of instinct, a few examples of them will 

 be now adduced. 



1 See Trans. Entomological Society of London, 1887, p. 24. 



