282 or THE BLOOD: ITS VITAL PROPERTIES, 



certain substance in the Blood, appears to determine the formation of the 

 particular tissue, of which that substance is the appropriate pabulum. And 

 thus, as the abstraction of the material required for each part leaves the 

 blood iu a state fitted for the nutrition of other parts, it seems to follow, as 

 Mr. Paget has further remarked, 1 that such a mutual dependence exists 

 among the several parts and organs of the body, as causes the evolution of 

 one to supply the conditions requisite for the production of another; and 

 hence, that the order in which the several organs of the body appear in the 

 course of development, while it is conformable to the law of imitation of the 

 parent, and to the law of progressive ascent towards the higher grade of 

 being, is yet the immediate result of changes effected in the condition of the 

 blood by the antecedent operations. And this view is confirmed by many 

 circumstances, which indicate that certain organs really do stand in such a 

 complemental relation to one another as it implies; a large class of facts of 

 this order being supplied by the history of the evolution of the generative 

 apparatus, and by that of the concurrent changes in other organs (especially 

 the tegumentary) which are found to be dependent upon it, although there 

 is no direct functional relation between them. Thus, the growth of the 

 beard in man at the period of puberty, is but a type of a much more im- 

 portant change which takes place in many animals with every recurrence of 

 the period of generative activity. If the development of the male organs be 

 prevented, the evolution of the beard does not take place; while the cessa- 

 tion or the absence of activity in the female organs is often attended by a 

 strong growth of hair on the face, as well as by other changes that may be 

 attributed to the presence of some special nutritive material in the blood, 

 for which there is no longer any other demand. This, again, shows itself 

 yet more strongly iu Birds ; among which (as Hunter long since pointed out 2 ) 

 it is no uncommon occurrence for the female, after ceasing to lay, to assume 

 the plumage of the male, and even to acquire other characteristic parts, as 

 the spurs in the fowl tribe. Moreover, it has been ascertained by the ex- 

 periments of Sir Philip Egerton, that if a buck be castrated while his 

 antlers are growing and are still covered with the "velvet," their growth is 

 checked, they remain as if truncated, and irregular nodules of bone project 

 from their surfaces; whilst, if the castration be performed when the antlers 

 are full grown, these are shed nearly as usual at the end of the season, but 

 in the next season are only replaced by a kind of low conical stumps. That 

 these and similar changes iu the development of organs are immediately de- 

 termined by the condition of the circulating fluid, that is, by the presence 

 or absence of the appropriate "pabulum" for the parts iu question, would 

 further seem likely from the fact, that they may be artificially induced by 

 circumstances which directly affect the condition of the blood. This has 

 been shown by Mr. Yarrell, 3 in regard to the assumption of the male plum- 

 age by the female; and a still more remarkable and satisfactory proof is 

 furnished by the conversion of the "worker" larva of the Bee into a perfect 

 "queen," solely through a change of diet. 4 



"2*20. Thus, then, the precise condition of the Blood at any one time, is 

 dependent upon a vast variety of antecedent circumstances, and can scarcely 

 be the same at any two periods of life, nor, indeed, in any two parts of its 

 course, even in one and the same individual. Yet we find that, taken as a 

 whole, it exhibits such a remarkable constancy in its leading features, that 

 we seem justified in the belief that, as Harvey and Hunter always maiu- 



1 Op. cit., p. 32. 



2 Account of an Extraordinary Pheasant, in Hunter's Works, Palmer's edit., vol. 

 iv, p. 44. 



3 Philosophical Transactions, 1827. 4 Princ. of Comp. Phys., \ 119. 



