34i LAWS OF VARIATION. Chap. XXVI. 



fattening well. With fowls which have large topknots and 

 beards the comb and wattles are generally much reduced in 

 size. Perhaps the entire absence of the oil-gland in fantail 



pigeons may be connected with the great development of their 

 tails. 



Mechanical Pressure as a Cause of 3 Modifications. —In some 

 few cases there is reason to believe that mere mechanical 

 pressure has affected certain structures. Every one knows that 

 savages alter the shape of their infants' skulls by pressure at 

 an early age ; but there is no reason to believe that the result 



the 



inherited. Nevertheless Vrolik and Weber 10 maintain 

 shape of the human head is influenced by the shape 

 of the mother's pelvis. The kidneys in different birds differ 

 much in form, and St. Ange u believes that this is determined 

 by the form of the pelvis, which again, no doubt, stands in 

 close relation with their various habits of locomotion. In snakes, 

 the viscera are curiously displaced, in comparison with their 

 position in other vertebrates ; and this has been attributed by 

 some authors to the elongation of their bodies : but here as 



10 Prichard, ' Phys. Hist, of Man- 1039. 



kind,' 1851, vol. i. p. 324. u Tj e ber Fotale Kachites, < Wiirz- 



■ 'Annales des Sc. Nat./ 1st series, burger Medicin. Zeitschrift,' 1860, B. i. 



torn. xix. p. 327. s. 265. 

 12 'Comptes Eendns,' Pec. 1864, p. 





>f 







in so many previous cases, it is impossible to disentangle any 

 direct result of this kind from that consequent on natural selec- 

 tion. Godron has argued 12 that the normal abortion of the 

 spur on the inner side of the flower in Corydalis, is caused by 

 the buds being closely pressed at a very early period of growth, 

 whilst under ground, against each other and against the stem! 

 Some botanists believe that the singular difference in the shape 

 both of the seed and corolla, in the interior and exterior florets 

 in certain compositious and umbelliferous plants, is due to the 

 pressure to which the inner florets are subjected ; but this con- 

 clusion is doubtful. 



^ The facts just given do not relate to domesticated produc- 

 tions, and therefore do not strictly concern us. But here is 

 a more appropriate case : H. Miiller ]3 has shown that in short- 



■ 



