296 
must have migrated forwards and backwards, changing 
their locality but not their climate, and remaining always 
“unchanged by changing place.” If, then, a given species 
could manage through the geological ages to find a sphere 
similar to the one to which it had been long ago adapted, 
there seems no reason why it should undergo modification, 
at any rate through the influence of external conditions. 
In the second place we learn from the experience of 
artificial selection that, while conditions remain unchanged, 
those variations which necessarily arise in connection with 
sexual reproduction do not lead to the establishment 
of new varieties, races, or species, apart from the principle 
of selection. Mr. Darwin illustrates this law by comparing 
the gold-fish and the carp. The former are kept in glass 
vessels and have been made the subjects of artificial 
selection and have therefore yielded many races. The 
carp have remained unmodified, because artificial selection 
could not be brought to bear upon them. These facts 
illustrate the general principle that, without the isolation 
of similar variants for breeding purposes, the variations 
which are necessarily associated with sexual reproduction 
will be swallowed up in the average of the race by virtue 
of the principle of regression to mediocrity, and thus 
the characteristic type of a species will be maintained. 
It is further very important to observe in this connection 
that a very great amount of individual variety is con 
sistent with the continued stability of species. Nor does 
the idea of the stability of species exclude the idea of 
a severe struggle for existence ; for it is almost impossible 
to conceive that the species which have undergone little or 
no change through all the æons of geological time can 
have been altogether free from the strife. And this 
probability becomes a certainty when we consider what 
actually takes place at the present time. When circum-