140 Proceedings of Royal Society of Edinhurgh, [jan. 30, 
alone, both in the case of the guinea-pig and dog. The rabbit and 
the cat certainly do not possess more highly developed brains, yet 
no such scratching movements can be elicited. The difference 
does not depend then upon a question of development either 
upwards or downwards, but rather upon a variation of habit or 
build. From increased mobility of the body, or from altered habits, 
the cat and rabbit may have come to use their eyes and head, 
whereby the brain is called into action, in place of the leg used by 
some ancestral type. Or, again, it is possible that the dog and 
guinea-pig may have acquired the use of the leg for scratching from 
altered habits, or from loss of mobility. 
JSTor is it difficult to explain the condition of things seen in the 
human subject. A child is born without such working connections 
between the sensory surfaces of its body and the corresponding 
groups of muscles as would lead to the approximation of a limb to 
any particular part of the skin. This comes only by laborious ex- 
perimentation on the part of the child. It sees its foot, and directs 
its hand to it. It feels the touch, and is conscious of the move- 
ment it has made. By continual practice it can touch most parts 
of its body. This is learnt only by experience, which has involved 
the use of sight, and therefore depends largely on the action of the 
brain. 
On this account, if the spinal cord be divided, we should not 
expect a man, upon having the calf of one leg tickled, to be able to 
Scratch it with the foot of the other leg, because during his extra- 
uterine development the brain was a necessary factor in producing 
such movements. Such is, indeed, the case, for although spasmodic 
jerks of muscles may be called forth by stimuli applied to the 
skin, an absence of purposive movements is noticed as soon as the 
cord is severed. 
8. Reply to Professor Boltzmann. By Prof. Tait. 
Hearing, again by accident, that Professor Boltzmann has in the 
Vienna Sitzungshericlite published a new attack on my papers about 
the Kinetic Theory, I at once ordered a copy, which has at length 
arrived. As my papers appeared in our Transactions, I think my 
answer to this fresh attack should be communicated in the first 
