264 Original Articles. | April, 
than it does in the Chick in ovo. Indeed, it is impossible to study 
the growth of any of the higher organisms,—which not merely con- 
sists in the formation of new parts, but also involves a vast amount 
of interstitial change—without perceiving that in the remodelling 
which is incessantly going on, the parts first formed must be removed 
to make way for those which have to take their place. And such 
removal can scarcely be accomplished without a retrograde metamor- 
phosis, which, as in the numerous cases already referred to, may be 
considered with great probability as setting free constructive force to 
be applied in the production of new tissue. 
If, now, we pass on from the intra-uterine life of the Mammalian 
organism to that period of its existence which intervenes between birth 
and maturity, we see that a temporary provision is made in the acts of 
lactation and nursing for affording both food and warmth to the young 
creature, which is at first incapable of adequately providing itself 
with aliment, or of resisting external cold without fostering aid. And 
we notice that the offspring of Man remains longer dependent upon 
parental care than that of any other Mammal, in accordance with the 
higher grade of development to be ultimately attained. But when the 
period of infancy has passed, the child that is adequately supplied 
with food, and is protected by the clothing which makes up for the 
deficiency of other tegumentary covering, ought to be able to maintain 
its own heat, save in an extremely depressed temperature ; and this it 
does by the metamorphosis of organic substances, partly derived 
from its own fabric, and partly supplied directly by the food, into 
binary compounds. During the whole period of growth and develop- 
ment, we find the producing power at its highest point; the circula- 
tion of blood being more rapid, and the amount of carbonic acid 
generated and thrown off being much greater in proportion to the bulk 
of the body, than at any subsequent period of life. We find, too, in 
the large amount of other excretions, the evidence of a rapid metamor- 
phosis of tissue; and it can hardly be questioned (if our general doc- 
trines be well founded) that the constructive force that operates in the 
completion of the fabric will be derived in part from the heat so 
largely generated by chemical change, and in part from the descent 
which a portion of the fabric itself is continually making from the 
higher plane of organized tissue to the lower plane of dead matter. 
This high measure of vital activity can only be sustained by an ample 
supply of food; which thus supplies both material for the construe- 
tion of the organism, and the force by whose agency that construction 
isaccomplished. How completely dependent the constructive process 
still is upon Heat, is shown by the phenomena of reparation in cold- 
blooded animals; since not only can thé rate at which they take place 
be experimentally shown to bear a direct relation to the temperature 
to which these animals are subjected, but it has been ascertained that 
any extraordinary act of reparation (such as the reproduction of a limb 
in the Salamander) will only be performed under the influence of a 
temperature much higher than that required for the maintenance of 
the ordinary vital activity. After the maturity of the organism has 
been attained, there is no longer any call for a larger measure of con- 
