1890.] Recent Literature. 925 
The question is, How are the small necessary and admitted differences 
stimulated to develop into the obvious differences recognized by syste- 
matic biologists? To this he answers that the reactions of the organ- 
ism against the physical forces and mechanical properties of its 
environment are abundantly sufficient, if we are granted a simple 
organism, with a tendency to grow, time for the operation of forces, 
and the principle of the survival of the fittest. 
It is often assumed that the possibility of variation is equal in every 
direction. A consideration of the dynamic conditions of life shows 
that this is not the case. Under conditions which would permit it, 
resulting organic forms would be sub-spherical, and would have to pass 
their entire existence in constant rotation. The moment one of them 
came to rest, it would be subjected to unequal stresses. Light, 
gravity, nutrition, etc., would be unequally distributed, forcing an 
unequal growth and specialization of regions. Inequality established, 
locomotion, with attendant frietion and resistance, would confirm the 
inequality. Organic matter, as such, is in no sense released from the 
servitude of matter to the operation of physical forces. 
Mr. Dall divides the operation of biologie selection into two cate- 
gories: ıst, That in which fitness and unfitness are determined by 
the perfection in adjustment of the individual to the mechanics of the 
environment; 2d, That in which intelligence becomes a factor. The 
latter includes sexual selection, mimicry, protective coloration, etc. 
It is not necessary that the organism which is modified shou ess 
even. consciousness; but one of the two parties to the modification 
must possess intelligence of a certain grade. It is probable that 
influences of the second category operate more rapidly and produce 
greater diversity in development than could have been expected from 
essentially sporadic. A pathologic incident may affect the progeny, 
but only in trifling numbers, and it is of no importance to the dynamic 
hypothesis whether it can be proven or not. The forces invoked by 
dynamic hypothesis, on the other hand, affect every individual of a race 
. and every generation as long as the environment continues unchanged. 
