894 The American Naturalist. [November, 
himself has retreated from this stronghold of his theory as he 
found it untenable. 
In reply to the criticism of Prof. Vines that it was “absurd 
to say that an immortal substance can be converted into a 
mortal substance,” Prof. Weismann without hesitation aban- 
dons the conception of molecular stability in the germ plasma, 
and presents his theory of heredity in a new form, that is more 
in accordance with physiological laws, and at the same time 
appears to be fatal to the assumptions made by his followers. 
He says, “does not life here as elsewhere depend on metabol- 
ism—that is to say a constant change of material? And what 
is it then which is immortal? Clearly not the substance but 
only a definite form of activity,”—“ An immortal unalterable 
living substance does not exist but only immortal forms of activity 
of organized matter.” The material continuity of the germ 
plasma is therefore discarded and replaced with the concep- 
tion of a mode of motion manifest in matter that is continually 
undergoing metabolic changes. 
As the complex molecular substance of the germ plasma is 
brought into intimate relations with the metabolism of the 
body plasma through its own metabolic activities, we can 
readily perceive how acquired habits of the organism in mod- 
ifying the general and special metabolism of the body must 
also have an influence on the substance of the germ cells, and 
through their constantly changing substance on the forms of 
activity, or modes of motion, that are transmitted from one 
generation to another in accordance with the new theory. It 
is then evident that the assumed independence of the germ 
cells of all influence from the surrounding body plasma, that 
is relied upon to prove the non-inheritance of acquired char- 
acters, derives no support from the present conditions of phys- 
iological science. 
There are many functional variations in the activities of 
the different organs of the body that can only be attributed 
to changes in the environment and food supply in connection 
with the habits of the individual, and they are so clearly 
defined and of such frequent occurrence that it seems to be 
unnecessary to assume fortuitous variations in the germ cells 
