818 The American Naturalist. [October, 
in the water, is the air vital to them? Aristotle and the older 
naturalists could not answer these questions. To them, on the 
respiratory side at least, all life was not in any sense the same. 
It was not until chemistry and physics were considerably 
developed, not until the air-pump, the balance and the burette 
were perfected that it was possible to give more than a tenta- 
tive answer. Not until the microscope could increase the 
range of the eye into the fields of the infinitely little, was 
it possible to form even an approximately correct conception. 
The first glimmerings of the real significance of respiration 
for all living things was in the observation that the air which 
would not support a flame, could not support life, although 
it might be breathed. That is, there must be something in 
the transparent air that feeds the flame and becomes the 
breath of life, the real pabulum vitæ, the merely mechanical 
action of the air not being sufficient. 
Since the experiments on insects and other animals with 
the air-pump by Boyle (1670), by Bernuilli on subjecting 
fishes to water out of which all the air had been boiled, and 
those of Mayow (1674), it became more and more evident that 
respiration was not confined to the higher forms but was a 
universal fact in the organic world. Then came the most 
fruitful discoveries of all, made by the immortal Priestley 
(1775-6), viz., that the air is not an element but composed of 
two constituents, nitrogen, which is inert in respiration, and 
oxygen, which is the real vital substance of the air, the sub- 
stance which supports the flame of the burning candle and 
` the life of the animal as well. 
What would seem more simple at this stage of knowledge 
than that the parallel between the burning candle and the 
living organism should be thought to represent truly the real 
conditions? That as the candle consumes the oxygen in burn- 
ing and gives out carbon dioxide, so the living thing breathes in 
oxygen and gives out in place of that consumed, carbon diox- 
ide. And as in each case heat is produced, what would be 
more natural than to look upon respiration as a simple com- 
bustion? This was the generalization of Lavoisier (1780-89). 
As he saw it, the oxygen entered the lungs, reached the blood 
