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 ivere 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 ;md 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. 
