XLVI PHILOSOPHICAL SOCIETY OF WASHINGTON. 
exercise important structural changes are produced when conjoined 
with the changes due to heredity. 
All these changes result in progress, from the fact that those indi- 
viduals whose change is in a direction out of harmony with the en- 
vironment ultimately perish, while those whose change is in a direc- 
tion in harmony with the environment survive. This method of 
adaptation or evolution in biology is called “ the survival of the 
fittest.” 
The rate of evolution by survival is greatly accelerated by another 
condition. Each pair of biotic bodies reproduce a large number of 
new bodies, so that reproduction from generation to generation is in 
a high geometric ratio. The earth having become occupied with 
all the biotic beings that can derive sustentation therefrom, but a 
small fraction of the beings produced in a generation can live. Few 
survive, many succumb. Survival by adaptation is therefore made 
more efficient by competition. 
There are other changes in the biotic kingdom brought about by 
adaptation. The multiplicity of biotic beings, causing over-popula- 
tion, has crowded them into every conceivable habitat—in the air, 
on the land, and in the water; and living beings have become 
adapted thereto by the development of wings, legs, fins, and correl- 
ative organs. Thus by exercise organs have been developed, and 
by non-exercise other organs have been atrophied, until living be- 
ings have become specialized for a vast diversity of habitats—for 
life on the mountain and in the valley, in the light and in the dark, 
in the cold and in the heat, in humid regions and in arid re- 
gions. Living beings have also been adapted to various kinds of 
food and to various methods of acquisition—in fine, to a great 
variety of conditions. 
This specialization by development, through exercise and non- 
exercise, must be clearly distinguished from the processes of evolu- 
tion. The heterogeneous living beings thus produced are but multi- 
plied and diverse forms, animals and plants alike being as often de- 
graded as evolved in the processes of specialization. Degradation 
is especially to be noticed in parasitic animals and others adapted 
to extremely abnormal habitats ; but it should be understood that 
a form thus produced may, in the process of its production and sub- 
sequent existence, make progressive change in the system of its 
structure by the methods of evolution already characterized. 
Specialization is greatly accelerated by a peculiar method. As 
