Chap. XXVI. SUMMARY. 345 



in the individuals of the same species ; we have also seen in 

 the earlier chapters of this work how variable secondary sexual 

 characters become under domestication. 



Summary of the three previous Chapters on the Laws of Variation. 



In the twenty-third chapter we saw that changed con- 

 ditions occasionally, or even often, act in a definite manner 

 on the organisation, so that all, or nearly all, the individuals 

 thus exposed become modified in the same manner. But 

 a far more frequent result of changed conditions, whether 

 acting directly on the organisation or indirectly through the 

 reproductive system, is indefinite and fluctuating variability. 

 In the three last chapters, some of the laws by which such 

 variability is regulated have been discussed. 



Increased use adds to the size of muscles, together with 

 the blood-vessels, nerves, ligaments, the crests of bone and 

 the whole bones, to which they are attached. Increased 

 functional activity increases the size of various glands, and 

 strengthens the sense-organs. Increased and intermittent 

 pressure thickens the epidermis. A change in the nature of 

 the food sometimes modifies the coats of the stomach, and 

 augments or decreases the length of the intestines. Continued 

 disuse, on the other hand, weakens and diminishes all parts 

 of the organisation. Animals which during many generations 

 have taken but little exercise, have their lungs reduced in 

 size, and as a consequence the bony fabric of the chest and 

 the whole form of the body become modified. With our 

 anciently domesticated birds, the wings have been little used, 

 and they are slightly reduced ; with their decrease, the crest 

 of the sternum, the scapulas, coracoids, and furculum, have all 

 been reduced. 



With domesticated animals, the reduction of a part from 

 disuse is never carried so far that a mere rudiment is left ; 

 whereas we have reason to believe that this has often occurred 

 under nature ; the effects of disuse in this latter case being 

 aided by economy of growth, together with the intercrossing 

 of many varying individuals. The cause of this difference 

 between organisms in a state of nature, and under domestica- 

 tion, probably is that in the latter case there has not been 



