336 LAWS OF VARIATION. Chap. XXVL 



Gucurbita pepo which bear large fruit yield a small crop, 

 according to Naudin; whilst those producing small fruit 

 yield a vast number. ' Lastly, I have endeavoured to show in 

 the eighteenth chapter that with many cultivated plants 

 unnatural treatment checks the full and proper action of the 

 reproductive organs, and they are thus rendered more or less 

 sterile ; consequently, in the way of compensation, the fruit 

 becomes greatly enlarged, and. in double flowers, the petals 

 are greatly increased in number. 



With animals, it has been found difficult to produce cows 

 which yield much milk, and are afterwards capable of fatten- 

 ing well. With fowls which have large top-knots and beards 

 the comb and wattles are generally much reduced in size ; 

 though there are exceptions to this rule. 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 Modifications. — In some 

 few cases there is reason to believe that mere mechanical 

 pressure has affected certain structures. Yrolik and Weber 9 

 maintain that the 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 10 believes that this 

 is determined by the form of the pelvis, which again, no 

 doubt, stands in close relation with their power 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 in so many previous cases, it is impossible to 

 disentangle a direct result of this kind from that consequent 

 on natural selection. Godron has argued u that the abortion 

 of the spur on the inner side of the flowers in Corydalis. is 

 caused by the buds at a very early period of growth whilst 

 underground being closely pressed against one another and 

 against the stem. Some botanists believe that the singular 

 difference in the shape both of the seed and corolla, in the 



9 Prichard, ' Phvs. Hist, of Man- series, torn xix. p. 327. 



kind.' 1851. vol. i. p. 324. " ; Comptes Reudus,' Dec. 1864. p 



10 -Aunales des So. Nat.,' 1st 1039. 



