i6 
The term “preservation” is obviously ambiguous, for it 
may mean the preservation of the best and most useful, 
and of these alone; or it may mean the preservation of the 
best, and most useful, in company with those which are 
not the best. Now we are told that both these kinds of 
“preservation” take place alike in Art and in Nature. 
It would facilitate the clear understanding of the sub 
ject if the two kinds of selection were to be distinguished 
by distinctive terms. We have to distinguish between 
preservation of the fittest alone, and preservation of the 
fittest and of some not the fittest. The latter is well 
expressed by the phrase “ elimination of the worst.” 
In order that Natural Selection should take place, there 
must be a change of external conditions, otherwise there 
would be no advantage in a modification of organism. 
There must also be a struggle for existence, or there 
could be no Natural Selection. It therefore becomes a 
question of considerable interest to ask whether or not 
external circumstances frequently change; and whether or 
not the struggle for existence is always going on, seeing 
that on these questions depends the continuous or the 
intermittent action of Natural Selection. 
Speaking of the change in external conditions, in his 
demonstration of the origin of species by Natural Selection, 
Mr. Wallace places on his list of proved facts : “Change 
of external conditions universal and unceasing.”* In his 
Darwinism he mentions: “Changes of climate which are 
continually occurring, owing either to cosmical or geo 
graphical causes,”-f- “under the ever-changing conditions 
both of the inorganic and organic universe.”* But else 
where he says: “Under conditions which do not perceptibly 
vary from year to year and from century to century.”^ 
* Contributions, p. J02. 
+ p. 22. i p. 122. § p. IOJ.