No. 467. MOMENTUM IN VARIATION. 8 
43 
be this tendency to carry them to excess, as in Titanotheres, 
Tinoceras, Elasmotheres, Irish elk, etc. | 
The above are selected examples in which a feature once use- 
ful has been developed beyond its maximum utility. Many 
others equally striking might be cited, the explanation of all of 
which is extremely difficult unless such a factor as momentum is 
called in. In the light of this factor, however, a logical and 
apparent cause is found. Momentum also explains why a char- 
acter that originated in accordance with the environment devel- 
ops so rapidly, and why, when an animal had reached adjustment 
to its surroundings, it still goes on beyond a perfect adjustment. 
It may be laid down as a rule then that a variation started along 
any line tends to carry that line of development to its ultimate, 
being driven by momentum. If the feature is detrimental the 
group dies out ; if, however, it is merely a minor feature it makes 
a handicap. A line of development may be stopped and its 
momentum overcome but the tendency is to keep right on. 
This factor of momentum has not been given the importance 
due it, although it is felt in the undercurrent of the thought of 
several authors. It is the writer's belief, however, that it should 
find an important place in the explanation of animal structures. 
AMHERST, Mass. 
