164 DEVELOPMENT AND HEREDITY. 



spongy part of bones shows that the bone septa 

 between the cavities are arranged along the lines 

 of greatest stress. To such causes must we ascribe 

 the length of the bones in the legs of quadrupeds 

 which run and do not use their legs for climbing, 

 digging, or swimming. The shock of the feet 

 striking the ground furnishes the motive for the 

 lengthening of the bones. In recognising this as 

 an efficient cause, we must not forget that other 

 forces, which cannot be defined, are at work limiting 

 and modifying the effects. 



The giraffe seems to present the most remarkable 

 illustration of the lengthening of the bones as the 

 result of the frequent repetition of such shocks. As 

 is well known, this animal feeds on the foliage of 

 trees. From the earliest youth of the species, 

 and the earliest youth of each individual, it must 

 have been stretching upwards for food, and, as 

 is the custom of such quadrupeds, it must have 

 constantly raised itself off its fore feet, and as it 

 dropped must have received a shock that made 

 itself felt from the hoofs through the legs and 

 vertical neck to the head. In the hind legs the 

 shock would not be felt. It is impossible to imagine 

 that an animal which, during the greater part 

 of every day of its life (both its individual and 

 racial life), performed motions so uniform and 



