Chap. XXIV. EFFECTS OF USE AND DISUSE. 295 



hydrocephalus the open and dangerous spaces in the skull are filled up with 

 new bones, which interlock by perfect serrated sutures. 8 But most physiolo- 

 gists, especially on the Continent, have now given up the belief in plastic 

 lymph or blastema, and Virchow 9 maintains that every structure, new or old, 

 is formed by the proliferation of pre-existing cells. On this view false mem- 

 branes, like cancerous or other tumours, are merely abnormal develop- 

 ments of normal growths ; and we can thus understand how it is that they 

 resemble adjoining structures ; for instance, that " false membrane in the 

 " serous cavities acquires a covering of epithelium exactly like that which 

 " covers the original serous membrane ; adhesions of the iris may become 

 " black apparently from the production of pigment-cells like those of the 



uvea 



No doubt the power of reparation, though not always quite perfect, is an 

 admirable provision, ready for various emergencies, even for those which 

 occur only at long intervals of time. 11 Yet this power is not more won- 

 derful than the growth and development of every single creature, more 

 especially of those which are, propagated by fissiparous generation. This 

 subject has been here noticed, because we may infer that, when any part 

 or organ is either greatly increased in size or wholly suppressed through 

 variation and continued selection, the co-ordinating power of the organisa- 

 tion will continually tend to bring all the parts again into harmony with 

 each other. 



On the Effects of the Increased Use and Disuse of Organs. 



It is notorious, and we shall immediately adduce proofs, that 

 increased use or action strengthens muscles, glands, sense-organs, 

 &c. ; and that disuse, on the other hand, weakens them. I have 

 not met with any clear explanation of this fact in works on 

 Physiology. Mr. Herbert Spencer 12 maintains that when muscles 

 are much used, or when intermittent pressure is applied to the 

 epidermis, an excess of nutritive matter exudes from the vessels, 

 and that this gives additional development to the adjoining 

 parts. That an increased flow of blood towards an organ leads 

 to its greater development is probable, if not certain. Mr. 

 Paget 13 thus accounts for the long, thick, and dark-coloured hair 

 which occasionally grows, even in young children, near old- 

 standing inflamed surfaces or fractured bones. When Hunter 



8 These cases are given by Blumen- vol. i., 1853, p. 357. 

 bach m his ' Essay on Generation,' pp. n Paget, idem, p. 150. 



52 > 54 - 12 t The Principles of Biology,' vol. 



9 'Cellular Pathology,' trans, by Dr. ii. 1866, chap. 3-5. 

 Chance, I860, pp. 27, 441. 13 « Lectures on Pathology,' 1853, 



10 



Paget, ' Lectures on Pathology,' vol. i. p. 71. 



