428 CORRELATION OF PHYSICAL AND VITAL FORCES. 



mammal than it does in the chick in ovo. Indeed, it is im 

 possible to study the growtn of any of the higher organisms 

 which not merely consists 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 metamorphosis, 

 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 tempo 

 rary 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 de 

 pendent 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, adequately supplied with food, and pro 

 tected by the clothing which makes up for the deficiency of 

 other tegumcntary 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 development, we find the producing power at 

 its highest point ; the circulation 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 



