6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



But in some cases Spencer is more specific. With 

 reference to the inequality in the basal lobes of the 

 erect leaves of Tilia and other plants, he says: 1 "A 

 considerable deviation from bilateral symmetry may 

 be seen in a leaf which habitually so carries itself that 

 the half on the one side of the midrib is more shaded 

 than the other half. The drooping branches of the 

 lime show us leaves so arranged and so modified. On 

 examining their attitudes and their relations one to 

 another, it will be found that each leaf is so inclined 

 that the half of it next the shoot grows over the shoot 

 and gets plenty of light ; while the other half so hangs 

 down that it comes a good deal into the shade of the 

 preceding leaf. The result is that having learned which 

 fall into these positions, the species profits by a large 

 development of the exposed halves ; and by survival 

 of the fittest acting along with the direct effect of extra 

 exposure, this modification becomes established.' 1 In 

 his discussion of the origin of the characters of ani- 

 mals, Spencer is also sometimes specific. Respecting 

 the development of muscular insertions he remarks : 2 

 "Anatomists easily discriminate between the bones of 

 a strong man and those of a weak man by the greater 

 development of those ridges and crests to which the 

 muscles are attached ; and naturalists on comparing 

 the remains of domesticated animals with those of 

 wild animals of the same species, find kindred differ- 

 ences. The first of these facts shows unmistakably 

 the immediate effect of function on structure, and, by 

 obvious alliance with it, the second may be held to do 

 the same, both implying that the deposit of dense sub- 

 stance capable of great resistance habitually takes 



\Op. cit., p. 143. 

 2 Loc. cit,, p. 200. 



