Chap. XXIV. EFFECTS OF USE AND DISUSE. 285 



Fallopian tubes, and falls into the abdomen, " nature pours out a 

 " quantity of plastic lymph, which forms itself into organised mem- 

 " brane, richly supplied "with blood-vessels," aud the foetus is 

 nourished for a time. In certain cases of 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 membranes, like cancerous or other 

 tumours, are merely abnormal developments of normal growths ; 

 aud we can thus understand how it is that they resemble adjoining 

 structures; for instance, that a "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." 10 



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

 an admirable provision, ready for various emergencies, even for such 

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

 wonderful than the growth and development of every single creature, 

 more especially of those which are propagated by fissiparous gene- 

 ration. 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 organisation will continually tend to 

 bring again all the parts into harmony with one another. 



On tlte 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. It has been experimentally proved by Eanke 12 that 

 the flow of blood is greatly increased towards any part which 

 is performing work, and sinks again when the part is at rest. 

 Consequently, if the work is frequent, the vessels increase in 

 size and the part is better nourished. Paget 13 also accounts 

 for the long, thick, dark-coloured hairs which occasionally 



8 These cases are given by Blumen- ls ' Die Blutvertheilung, &c. cler 

 bach in his ' Essay on Generation,' pp. Organe,' 1871, as quoted by Jaeger, 

 52, 54. ' In Sachen Darwin's,' 1874, p. 48. 



9 ' Cellular Pathology,' trans, by See also H. Spencer, ' The Principles 

 Dr. Chance, 1860, pp. 27, 441. of Biology,' vol. ii. 1866, chap. 3-5. 



10 Paget, 'Lectures on Pathology,' 13 ' Lectures on Pathology,' 1853_ 

 vol. i., L853, p. 357. rol. i. p. 71. 



11 Paget, ibid., p. 150. 



