Chap. XXIV. LAWS OF VARIATION '. NISUS FORMATIVUS. 28ci 



CHAPTER XXIV. 



LAWS OF VARIATION — USE AND DISUSE, ETC. 



NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OF THE ORGANISATION— 

 ON THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS — 

 CHANGED HABITS OF LIFE — ACCLIMATISATION WITH ANIMALS AND PLANTS 

 — 'VARIOUS METHODS BY WHICH THIS CAN BE EFFECTED — ARRESTS OF 

 DEVELOPMENT — RUDIMENTARY ORGANS. 



In this and the two following chapters I shall discnss, as well 

 as the difficulty of the subject permits, the several laws which 

 govern Variability. These may be grouped under the effects 

 of use and disuse, including changed habits and acclimatisation 

 — arrest of development — correlated variation — the cohesion 

 of homologous parts — the variability of multiple parts — com- 

 pensation of growth — the position of buds with respect to the 

 axis of the plant — and lastly, analogous variation. These 

 several subjects so graduate into one another that their dis- 

 tinction is often arbitrary. 



It may be convenient first briefly to discuss that co- 

 ordinating and reparative power which is common, in a higher 

 or lower degree, to all organic beings, and which was formerly 

 designated by physiologists as nisus formativus. 



Blumenbach and others 1 have insisted that the principle which 

 permits a Hydra, when cut into fragments, to develop itself into 

 two or more perfect animals, is the same with that which causes 

 a wound in the higher animals to heal by a cicatrice. Such cases 

 as that of the Hydra are evidently analogous to the spontaneous 

 division or fissiparous generation of the lowest animals, and like- 

 wise to the budding of plants. Between these extreme cases and 

 that of a mere cicatrice we have every gradation. Spallanzani,- by 

 cutting off the legs and tail of a Salamander, got in the course of three 

 months six crops of these members ; so that 687 perfect bones were 

 reproduced by one animal during one season. At whatever point 



1 ' An Essay on Generation,' Eng. 209. 

 translat., p. 18; Paget, ' Lectures on 2 'Au Essay on Animal Reproduc- 



Surgieal Pathology,' 1853, vol. i. p. tion,' Eng. translat., 17<J9, p. 79. 



