300 



LAWS OF VARIATION. 



Chap. XXIV 



vious ; and perhaps the liver may be affected by 

 and artificial food on which they largely subsist. 



>wn 



from being forced to transmit more blood, increase in diameter ; and this 

 increase cannot be accounted for by mere extension, as their coats gain in 

 strength. Mr. Herbert Spencer 24 has argued that with plants the flow of 

 sap from the point of supply to the growing part first elongates the cells 

 in this line ; and that the cells then become confluent, thus formino- the 

 ducts ; so that, on this view, the vessels in plants are formed by the mutual 

 reaction of the flowing sap and cellular tissue. Dr. W. Turner has re- 

 marked, 25 with respect to the branches of arteries, and likewise to a certain 

 extent with nerves, that the great principle of compensation frequently 

 comes into play ; for " when two nerves pass to adjacent cutaneous areas 

 " an inverse relation as regards size may subsist between them ; a deficiency 

 " in one may be supplied by an increase in the other, and thus the area of 

 " the former may be trespassed on by the latter nerve." But how far in 



iff 



gmal variation, and how far to increased use or action, is not clear. 



In reference to glands, Mr. Paget observes that " when one kidney is 

 " destroyed the other often becomes much larger, and does double work." 26 

 If we compare the size of the udders and their power of secretion in cows 

 which have been long domesticated, and in certain goats in which the udders 

 nearly touch the ground, with the size and power of secretion of these organs 

 in wild or half-domesticated animals, the difference is great. A good cow 

 with us daily yields more than five gallons, or forty pints of milk, whilst 

 a first-rate animal, kept, for instance, by the Damaras of South Africa, 2 ' 

 « rarely gives more than two or three pints of milk daily, and, should her 

 " calf be taken from her, she absolutely refuses to give any." We may 

 attribute the excellence of our cows, and of certain goats, partly to 

 the continued selection of the best milking animals, and partly to the 



inherited effects of the increased action, through man's art, of the secreting 

 glands. 



It is notorious, as was remarked in the twelfth chapter, that short-sight 

 is inherited; and if we compare watchmakers or engravers with, for 

 instance, sailors, we can hardly doubt that vision continually directed 

 towards a near object permanently affects the structure of the eye. 



Veterinarians are unanimous that horses become affected with spavins, 



rm 



and they are almost equally unanimous that these injuries are transmitted. 

 Formerly horses were not shod in North Carolina, and it has been asserted 

 that they did not then suffer from these diseases of the legs and feet. 28 



24 'Principles of Biology,' vol. ii.p. 263. 



1 



.----. Africa,' p. 318. For analogous cases in 

 - 5 « Natural History Eeview,' vol. iv., South America, see Aug. St. Hilaire, 

 Oct 1864, p. 617. 'Voyage dans le Province de Goyaz' 



26 ' Lectures on Surgical Pathol og y , ' 

 1853, vol. i. p. 27. 



27 Andersson, < Travels in South 



torn. i. p. 71. 



28 Brickelfs 'Nat. Hist, of North 

 Carolina/ 1739, p. 53. 



. 



I 



pi 



Bl 



a 



ab 



bl 



