ORGANIC EVOLUTION RECONSIDERED, 181 
other while varieties are so invariably; and it is this that, in 
the opinion of Romanes and many others, has never been ade- 
quately explained. 
Admitting that the theories of Romanes, Brooks, and Weis- 
mann have advanced us on the way to more complete views of 
the mode of origin of the forms of the organic world, it must 
still be felt that all theories yet propounded fall short of being 
entirely satisfactory. It seems to us unfortunate that the sub- 
ject has not received more attention from physiologists, as 
without doubt the final solution must come through that sci- 
ence which deals with the properties rather than the forms 
of protoplasm ; or, in other words, the fundamental principles 
underlying organic evolution are physiological. But, in the 
unraveling of a subject of such extreme complexity, all sci- 
ences must probably contribute their quota to make up the 
truth, as many rays of different colors compounded form white 
light. As with other theories of the inductive sciences, none 
can be more than temporary; there must be constant modifi- 
cation to meet increasing knowledge. Conscious that any 
views we ourselves advance must sooner or later be modified 
as all others, even if acceptable now, we venture to lay before 
the reader the opinions we have formed upon this subject as 
the result of considerable thought. 
All vital phenomena may be regarded as the resultant of 
the action of external conditions and internal tendencies. Amid 
the constant change which life involves we recognize two 
things: the tendency to retain old modes of behavior, and the 
tendency to modification or variation. Since those impulses 
originally bestowed on matter when it became living, must, in 
order to prevail against the forces from without, which tend 
to destroy it, have considerable potency, the tendency to modi- 
fication is naturally and necessarily less than to permanence of 
form and function. 
From these principles it follows that when an Amceba or 
kindred organism divides after a longer or shorter period, it is 
not in reality the same in all respects as when its existence 
began, though we may be quite unable to detect the changes; 
and when two infusorians conjugate, the one brings to the other 
protoplasm different in molecular behavior, of necessity, from 
having had different experiences. We attach great importance 
to these principles, as they seem to us to lie at the root of the 
whole matter. What has been said of these lower but inde- 
gs 
