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 



