MUTUAL DEPENDENCE OF PARTS OF ORGANISM. 319 
developed, their nutrition is kept np by the supply of their 
respective materials which they derive from the blood. Each 
tissue draws from the circulating current that which it re¬ 
quires ; and it is one of the most wonderful proofs of the skill 
with which the entire fabric has been planned and constructed, 
that the composition of the blood should be maintained at a 
nearly uniform standard, in spite of the continual change which 
is thus taking place in its actual components. It has been 
justly remarked, that each part of the body, by taking from 
the blood the peculiar substances which it needs for its own 
nutrition, does thereby act as an excretory organ, inasmuch 
as it removes from the blood that which, if retained in it, 
would be injurious to the nutrition of the body generally. 
383. Hence it seems that such a mutual dependence must 
exist 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 this view is borne out by 
a great number of phenomena of very familiar occurrence, 
which show that a periodical change in one set of organs 
governs changes in others which at first sight might seem to 
have no relation to them. Thus the plumage of Birds, at the 
commencement of the breeding season, becomes (especially in 
the male) more highly coloured, besides being augmented by 
the growth of new feathers ; but when the generative organs 
pass into their condition of periodical inactivity, the plumage 
begins at once to assume a paler and more sombre hue, and 
many of the feathers are usually cast, their nutrition being no 
longer kept up. So, again, it is no uncommon occurrence 
among Birds, for the female, after ceasing to lay, to assume 
the plumage of the male, and even to acquire other character¬ 
istic parts, as “ spurs ” in the fowl tribe. That, in these and 
similar instances, the development of organs is immediately 
determined by the presence or absence in the blood of the 
appropriate pabulum for the parts in question, and that its 
existence depends upon changes taking place in other parts, 
has been rendered still more probable by the results of expe¬ 
riments, which show that if the ordinary changes in one set 
of organs be prevented by their removal, those usually taking 
place in the others do not occur. 
384. Though all the tissues derive the materials of their 
development from the blood which circulates in the vessels, 
