151 
Laboratory Note. 
pairs remaining unaltered. The evidence from those parallel 
mutations with similar linkage relationships shows that even the 
finer elements of the germ-plasm have maintained their relative 
positions and potentialities from species to species. This indicates 
a degree of static fixity combined with mutation in single elements 
which is truly remarkable, indicating that even the finer details of 
structure in the germ-plasm may maintain their spatial relationships 
for long periods. A number of the mutations in D. virilis have not 
appeared in D. melanogaster, though it does not follow that the 
latter species is incapable of producing them. At present it is quite 
unknown why one mutation rather than another appears at any 
given time, though the evidence is strong that each represents an 
actual transformation of a germinal element, and not merely a loss. 
Clearly, the conception of parallel or homologous mutations is 
destined to be a very useful one in germinal analysis and in the 
study of relationships. In the next chapter we will consider its 
application to the study of variations in wild species. 
LABORATORY NOTE. 
NOTE ON AN IMPROVED METHOD FOR 
DEMONSTRATING THE ABSORPTION OF OXYGEN 
IN RESPIRATION. 
1 HAVE for some time been dissatisfied with the method which, 
I believe, is most generally employed in lectures and practical 
classes for demonstrating the fact that germinating seeds remove 
oxygen from the air by the process of respiration. 
The apparatus, as figured in Darwin and Acton’s “ Physiology 
of Plants” (Fig. 15) or in Keeble’s “Practical Plant Physiology,” 
consists of a filtering flask fitted with a cork and containing 
germinating seeds and a test-tube with strong caustic potash 
solution to absorb the carbon dioxide produced. To the side tube 
of the filtering flask is attached a long vertical glass tube, the lower 
end of which dips into a dish containing mercury. The carbon 
dioxide produced by the respiring seeds is absorbed by the caustic 
potash, and that some oxygen is used up in the process is shown by 
the mercury rising in the vertical tube. 
The objection to this form of apparatus is that the vertical 
column of mercury acts principally as a manometer registering a 
decrease in pressure, and the level of the mercury rises only a very 
few inches. So the fact that the germinating seeds are capable of 
using up in respiration all or nearly all of the oxygen in the flask, 
amounting approximately to one-fifth of the volume, is insufficiently 
demonstrated by the experiment. 
I would suggest the slight modification in the apparatus shown 
in the accompanying figure, which, I think, requires but little 
explanation, as an improved method of performing the experiment. 
In practice it has been found to give good results. 
