220 HAECKEL 



the fish. is. That was recognised long ago by Linne, 

 who assigned them a corresponding rank. The 

 fishes are the lowest group of the vertebrates ; the 

 frogs belong to the group immediately above them. 

 Now let us see how one of these frogs is developed 

 to-day. The frogs are oviparous (egg-laying) 

 animals. The mother frog lays her eggs in the 

 water, and in the ordinary course of nature a new 

 little frog develops from each of these eggs. But 

 the object that develops from them is altogether 

 different from the adult frog. 



This object is the familiar tadpole. At first it 

 has no legs, but it has a long oar-like tail, 

 with which it can make its way briskly in the 

 water. It breathes in the water by means of gills 

 just like a fish. It is only when the tadpole grows 

 four legs, loses its tail, closes up the gills at its 

 throat, and begins to breathe by the mouth and 

 lungs instead, that it becomes a real frog. There 

 can be no doubt whatever that the tadpole is very 

 much more like the fish in all the most important 

 particulars than the frog. Between the frog-egg 

 and the frog itself we have a stage of development 

 in each individual case of which we might almost 

 say that the young frog has first to turn into a 

 fish before it can become a frog. 



How are we to explain this ? 



At first people supposed something like the 

 following : All beings in nature are admirably 

 adapted to their environment and their life-con- 

 ditions. Whatever be the explanation of it, it is 

 a simple fact. Now, the frog lays its eggs in the 



