236 NATURAL SCIENCE [October 
The general truth is that when the habits and conditions are 
different at different periods of the individual life, then, and to a 
proportionate degree, will the structure of the individual be different 
during those periods. It may be said that this is merely another 
way of stating the principle of adaptation as the result of natural 
selection, the most advantageous variations being selected at each 
stage of life separately. But my contention is that, if there is not 
sufficient evidence of the occurrence, apart from the influence of 
habits and conditions, of the variations necessary to explain the 
other two kinds of difference, still less have we proof that the 
changes in the structure of the individual at different periods of 
life have been independent of the direct influence of the changed 
conditions. To my mind the phenomena of metamorphosis can 
only be explained on the principle that the different conditions 
acting on the individual at different periods of its life give rise to 
and determine the direction of the modifications which characterise 
the successive stages of the individual structure. 
This matter again can only be studied in actual instances. 
The most familiar case is that of the frog and other Amphibia. We 
can have no doubt that the air-breathing Amphibia were evolved 
from fishes, though we may not be able to say exactly what kind of 
fishes. We have, however, various transitional or intermediate 
forms in the lower Amphibia and in the Dipnoi or lung-fishes, which 
breathe air to some extent. Now, how can we conceive the con- 
version of a single individual fish into an air-breathing creature, 
apart from the change of conditions, the breathing of air? It is 
true that the blood can and does secrete gases, oxygen and other, 
into a closed air-bladder, but the structural arrangements connected 
with the action of lungs, cannot be conceived apart from the 
respiration of atmospheric air. We know of plenty of cases in 
which, the water being scarce or foul, fish have become capable of 
breathing air, in one way or another, but we have no evidence of the 
occurrence of variations in adult life tending towards air-breathing 
structures in fishes which are never exposed to the air. We do not 
find them, for instance, in fishes that live on the sea-bottom or in 
the ocean abysses. When the fish is exposed to the air at a late 
stage of its life, then its structure undergoes modification, first into 
a lung-fish breathing both air and water, or into an amphibian that 
retains its gills throughout life. Afterwards such a form spreads 
into places where water is still scarcer, and it becomes still more 
modified, so as to breathe air altogether, and to crawl about on 
land. 
But, at the same time, the young aquatic stage or larva is being 
modified. If we suppose that the tadpole resembles the ancestor of 
the frog, it follows that that ancestor was destitute of paired limbs 
