THE CAUSATION OF DISEASE. 1 43 



as the 'primary cause of natural variations. Although I can see 

 no escape from this conclusion, yet it is as well to note the fact 

 that Darwin always wrote most guardedly concerning the causes 

 of variability ; and he evidently regarded it as quite impossible 

 to explain variation on any general principle. The causes of 

 variability are grouped by him under the following heads : * — 



(a) Conditions of life. 



(b) Habit. 



(c) The use and disuse of parts. 

 (W) Correlation. 



(e) Inheritance. 



Now it seems to me that the causes of variations, as above 

 grouped, all operate through peculiarity of E, and this is a 

 convenient place to make good that assumption. Let us, 

 therefore, consider briefly these several causes seriatim. 



(a) Conditions of Life. — By this Darwin means the ex- 

 ternal forces to which the organism is exposed, such as food 

 (this when taken into the body is a force derived from without), 

 climate, &c. The nature of the food, both as regards quality and 

 quantity, undoubtedly influences structure, and, inasmuch as 

 different animals of the same species differ in regard to their 

 food supply, they tend to vary. In such a case, of course, 

 the external E — " food " — makes itself felt by modifying the 

 internal E. Numerous examples of the influence of food and 

 climate might be given. f 



(b) Habit. — " Change of habits," says Darwin, " produces 

 an inherited effect," as in the period of the flowering of 

 plants when transported from one climate to another. This 

 effect is obviously due to the altered E. The rhythm of the S 

 is altered in response to an alteration in the rhythm of the E. 

 The human organism may in similar fashion be educated to a 

 different rhythm. A man may be taught to sleep by night or day, 

 or at intervals of a few hours, and the organism may be so 

 educated that defecation shall take place at equal intervals of 

 time. There are many such rhythms, and they are of the utmost 



* Crossing is an important cause of variations, but this is included under 

 (e) Inheritance. 



f " Animals and Plants under Domestication," vol. ii. p. 243 (second edit, 

 revised). 



