52 EVOLUTION 



embryonic life. They never cut the gum, 

 they are absorbed at a very early stage, they 

 are not of the slightest use. It appears to us 

 that in the inheritance of the baleen whale 

 there must be definite " representative par- 

 ticles " corresponding to the typical mam- 

 malian dentition, that they are still strong 

 enough to insist on some expression in 

 development, and that so far as teeth are 

 concerned the whalebone whale is, therefore, 

 recapitulating, obviously in much condensed 

 form, an ancestral condition. 



A fish has a two-chambered heart, with an 

 auricle that receives impure blood from the 

 body and a ventricle that drives it to the gills. 

 In amphibians the auricle is divided length- 

 wise by a partition, so that the heart becomes 

 three-chambered. In reptiles the ventricle 

 is partially divided by a similar partition, and 

 this becomes complete in the case of the 

 crocodile. In birds and mammals the heart 

 of the adult is four-chambered, with two 

 auricles and two ventricles. But when we 

 inquire into the development of the heart 

 of the bird or of the mammal, we find a series 

 of stages which are in a general way parallel 

 to the historical evolution of the heart as we 

 see it registered in the successive grades fish, 

 amphibian and reptile. The same impression 



