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On the other hand, the laws which govern the degeneration 
and disappearance of a character are :— 
1.—Decreased use—the character being less serviceable 
requires a less amount of nutriment; and a less 
amount of nutriment being received the character 
is not elaborated in proportion to the other charac- 
ters of the organism. 
2.—Economy—the character being less serviceable or 
ornamental it is deprived of nutriment in order 
that such nutriment may be more advantageously 
used elsewhere. 
3.—The transmission of the character in accordance with 
the laws of heredity laid down, and with the laws 
which regulate the transmission of variation. 
It may be remarked that among Ammonites progressive 
features occur in a definite successional order; while retro- 
gression which may commence at any time is the reversal of the 
order of progressive acquirement—an individual or a phylo- 
genetic series as it grows older, losing the progressive features 
one by one, in the reverse order to which they were acquired. 
There may also be retrogression in certain features and pro- 
gression in others going on at the same time; and, further, 
retrogression and progression may be alternately shewn in the 
phylogenetic series. 
When retrogression and progression go on together, the 
loss of features, in the reverse order to which they were 
acquired in progression, takes place of course only in relation to 
the part affected by the retrogression, or rather, degeneration. 
I shall notice the operation of this important law in the case 
of the hair of Man—the hair of the body being in a progressive 
state, and the hair of the head retrogressive: it is the hair 
of the head which degenerates in reverse order to acquirement 
—that is, “the last to come is the first to go.” This is, in fact, 
the law of retrogression, only it is necessary to distinguish 
_ local retrogression from general retrogression: 
Local retrogression may be due to alteration of habits 
and environment, or may be the economical accompaniment of 
