1892.] Heredity of Acquired Characters. 899 
of 158° with apparent satisfaction, and exhibiting a normal 
exercise of their nutritive and reproductive functions, were 
however killed when subjected to a temperature of 60°, which 
was the most favorable for their ancestors. 
The acquired habit of adjusting their physiological activities 
to an abnormally high temperature was undoubtedly trans- 
mitted through many thousand generations, and it is evident 
that the germ plasma was affected by the changes in the envi- 
ronment, either directly, or with greater probability through 
the modified metabolism of the body plasma. 
These experiments clearly indicate the importance of time, 
in some species at least, as a factor in the complete adjustment 
of even functional activities to changes in the environment. 
Seven years of persistent effort was required to bring about a 
change in the habits, or metabolic processes of these organisms 
that enabled them to endure, or actually enjoy, the final tem- 
perature of 158°, and a much longer time was evidently 
needed to produce any marked morphological changes. 
The transformations of energy in the metabolic processes of 
nutrition appear to be probable causes of variation, and possi- 
ble factors in evolution that require investigation. The effects 
of use and disuse are not obvious in many organs of an obscure 
nature and undetermined function, some of which may have 
intimate relations with the dynamic factors of nutrition, and 
thus serve a useful purpose which we are now unable to per- 
ceive. 
What are the relations of the so-called ductless glands, like 
the thyroid and the supra-renal capsules, to the utilization 
and conservation of energy? Are not the polar bodies of the 
ovum, and the thymus of the embryo temporary organs to 
transfer and conserve energy under special conditions that dis- 
appear at later stages of development? 
What molecular, or other changes take place in the organ- 
ism to bring about an intense activity of special functions, 
involving a more complete utilization of energy, as in in- 
creased milk production, or in improved fattening qualities? 
Questions like these must be answered, to furnish a satisfac- 
tory explanation of biological activities, and theories of nutri- 
