Chap. XXIV. LAWS OF VARIATION : NISITS FORMATIVUS. 293 



CHAPTEE XXIV. 



LAWS OF VARIATION — USE AND DISUSE, ETC. 



NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OP 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 discuss, 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 — 

 arrests of development — correlated variation — the cohesion of 

 homologous parts— the variability of multiple parts — compensa- 

 tion of growth— the position of buds with respect to the axis 

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

 subjects so graduate into each other that their distinction 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 desig- 

 nated by physiologists as the 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 with the spontaneous division or fissiparous generation 

 of the lowest animals, and likewise with the budding of plants. Between 

 these extreme cases and that of a mere cicatrice we have every gradation. 

 Spallanzam, 2 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 



i 'An Essay on Generation,' Eng. 2 . An E on Animal Reproduc- 



tions at., p. 18 ; Paget, 'Lectures on tion/ Eng. translat, 3769, p. 79. 

 Surgical Pathology,' 1853, vol. i. p. 209 



