20 SEXUAL DIMORPHISM 



to say exactly what kind of fishes. We have, however, various 

 transitional or intermediate forms in the lower amphibia and 

 the dipnoi or lung-fishes, which breathe air to some extent. 

 Now how can we conceive the conversion 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 or 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, and it is 

 converted 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 and of fin-rays; and that the 

 terrestrial form of limb, transversely jointed into three 

 segments, and divided at the extremity into five digits, was 

 not evolved from the fin of a fish, but was a new organ. 

 Such a view is very improbable and by no means inevitable. 

 It is much more reasonable to suppose that the terrestrial 

 limb was evolved by the modification of the fin of a fish. 

 The tadpole has lost its limbs, because in this temporary stage 



