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features which are further elaborations of former features, and 
when such developments indicate greater and greater vigour, 
the inference is that the members of such a series are in a 
progressive state. On the other hand, when a phylogenetic 
series shews continued reduction and loss of those features by 
which it was formerly distinguished, and when such reduction 
is accompanied by appearances of decreased vigour, the mem- 
bers of such a series are assumed to be retrogressive or 
degenerating. 
It may be advisable to briefly consider the causes which 
would affect progression and retrogression. 
When vitality is maintained by the lowest possible 
expenditure of force, and there remains over a considerable 
surplus of force beyond what is required to sustain life, 
progression would be the result; while under opposite condi- 
tions there would be retrogression. 
Change in the temperature, and change in the composition 
of the medium by which the organism is surrounded may 
entail upon it a greater or less expenditure of force in order 
to maintain life, and so may affect it favourably or the reverse. 
Similarly the quantity and quality of the food supply, and the 
difficulty or ease with which it is obtained entail a greater 
or less expenditure of force favouring retrogression or pro- 
gression as the case may be. 
Without, however, assuming any great change in the 
environment of an organism, the laws which would govern the 
appearance or elaboration of a new character may be stated 
as :— 
1.—Increased use—the character is of advantage to its 
possessor for use or ornament. Increased use 
means that more than its due share of nourishment 
must be received by the part used, and greater 
nourishment means increased proportionate growth. 
2.—The transmission of the character in accordance with 
the laws of heredity laid down, and with the laws 
which regulate the transmission of variation. 
