202 Inheritance of Acquired Characters 
change in their former manner of life, and in whom 
consequently the whole organism will be modified through 
functional adaptation. In this way may be explained 
the transformation of only one group of the individuals 
constituting a species into a new species, while the others 
remain unchanged. 
These few examples, even though so briefly outlined, 
are nevertheless quite enough to show us that the 
Lamarckian theory is capable of explaining at the same 
time both the evolution and the fixity of a species. But 
how can Weismann account for the inalterability and 
constancy of a given species? It goes without saying 
that he has no hesitation in attributing it, like variation 
itself, to natural selection again. But even if one were 
willing to suppose the environment immutable, is it 
possible that any species could ever come to such a 
degree of perfection in relation to its environment that 
every new variation in any direction whatever must make 
the conditions of this species worse and make its mem- 
bers less likely to be victorious in the struggle for 
existence? Is it not much more probable that however 
high a degree of adaptation to its environment a species 
may have attained, it can always become even better 
equipped for the struggle for existence through further 
transformations in certain directions, and consequently 
offer still greater opportunities for natural selection 
which is everywhere and always upon the qui vive? 
We must nevertheless be careful, in relation to this 
question of the fixity of species, not to attribute to the 
arguments which we have just set forth any greater value 
than they actually possess, especially because we know 
nothing, or only a very little, concerning the immediate 
circumstances that have actually existed in the develop- 
