88 The American Naturalist. [January, 
of contemporaneons adult individuals of the same species, thus lead- 
ing to a tendency toward concurrent or simultaneous variation of 
offspring in the same or a similar direction. 
It will be seen that this is the only hypothesis which renders the 
possibility of concurrent or simultaneous variation within the limits of 
a species either conceivable or intelligible. It also lends itself to an 
intelligible comprehension of the phenomena of the correlation of the 
growth of parts, and it is also the only view which holds out any 
promise of coérdination with the highly ingenious and suggestive 
hypotheses of Prof. Wilhelm Roux.! 
It will be at once perceived that my hypothesis of the acquisition 
of variations and their transmission is the simplest that has yet been 
offered. It interjects nothing hypothetical into our conception of the 
physical substratum of living organisms, except the necessarily un- 
known and unknowable constitution of the molecular factors of meta- 
bolism, already assumed by all scientific physiologists, all of whose 
conceptions of living processes are based upon the theory of meta- 
bolism, and thus brought into harmony with the all-inclusive doctrine 
of the conservation of energy. Growth and development without 
accompanying metabolism is simply unthinkable. All of the tend- 
encies, capabilities and manifestations of growth in all living organ- 
isms are coéxtensive with and the concomitants of metabolism. From 
this conclusion there is no possible means of escape. To imagine the 
existence in living bodies of a hypothetical entity for the sole and 
express purpose of superintending and ordering the sequence and 
modes of action of the processes of development is, to the mind of one 
who is imbued with the true scientific spirit, no better than an appeal 
to ‘‘vitality’’ to explain the sequence and nature of the phenomena 
of life. Such methods in biological science ought to have received 
their quietus from Huxley’s suggestive and witty comparison of the 
attested by the notorious transmissibility of the tendency to obesity in 
the human race, since illustrations will immediately occur to almost 
every one of families in which the tendency is known to be hereditary. 
Similarlv there is scarcely a possibility of doubt that the greatly in- 
creased fertility of domesticated races of animals and plants is almost 
exclusively due to a gradually increased power of appropriating nutri- 
ment due to a change in the molecular habit or mode of metabolism 
of the plasma of the body under better conditions of nourishment 
* Der Kampf der Theile im Organismus. 8vo., pp. VII., 244, Leipzig, 1881. 




