MOMENTUM IN VARIATION. 
F. B. LOOMIS. 
Ever since the theory of evolution was grounded and the fact 
that each form and species is the result of accumulated variations 
from a less specialized ancestor, was established, this same vari- 
ation has been scrutinized to find its causes and laws. Darwin 
made it more or less sporadic, and saw in the struggle for exist- 
ence the factor which selected the useful variations and eliminated 
the less suitable innovations. Later, recognizing many features, 
especially ornamental characters, as not useful, he also proposed 
sexual selection as a second factor in determining the preserva- 
tion of variations. Others, especially paleontologists, have been , 
struck by the fact that variations are in a line, so that useful 
features develop rapidly; and second, by the frequency with 
which several lines of variation appear in a form practically 
simultaneously, all in conformity to the particular habitat and 
habit of the possessor. Further, remarkable convergences are 
found in animals of different groups when placed in the same 
habitat. These facts lead to two lines of thought: first, that 
the environment acts on the animal more than merely to select 
such variations as chance may offer; and second, that the ani- 
mal in its life, by its habits, acquires characters which are handed 
to its offspring accentuated, and thus adaptive features are rap- 
idly developed. ! ; 
. In the above an explanation is found for the development of 
useful characters, but there remain still two classes to be 
accounted for. First, those which in their early stages are of 
no account, but when developed become important (as the cusps 
on teeth); and second those which have developed to perform a 
useful function but in later forms have reached a degree of spe 
cialization which has proved either a hindrance or the destruction 
of the possessor. To this class also belong those features devel- 
oped to a degree of perfection beyond the requirements of utility 
(sponge spicules, Radiolarian shells, some plant blossoms). . 
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