THE EVOLUTION OF MAN 



than an embryo set free. * And from the 

 genetic law, which recognizes in the embryo the 

 portrait of its ancestor, we conclude therefore 

 that newts and frogs are descended from crea- 

 tures which breathed through gills, that is to say, 

 descended from fish, since they are the only 

 vertebrates from which we may choosex 



But if these newts and frogs, according to the 

 assumption that we made a while ago, are noth- 

 ing but a side line of that main group from 

 which mammals also developed once upon a 

 time, nothing remains for us but to assume that 

 this main group in its entirety leads back to a 

 preceding station of water animals breathing 

 through gills. 



Some one may object and ask how it is that 

 no other animals besides frogs and newts, say, 

 for instance, reptiles, birds and mammals up to 

 man, have preserved breathing through gills in 

 the embryonic stage. Why does not a young 

 human being first become a tadpole before it be- 

 comes a man? Well, in the first place, the bio- 

 genetic law is not absolute. Very often it shows 

 itself only in dim outlines. On account of subse- 

 quent adaptation for purposes of protection, or 

 for other reasons, some of those reversions to 

 type have been subsequently eliminated. The 



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