EVIDENCES OF EVOLUTION 51 



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 rlevplopippnt. of til? h^P- r ^ 

 pf the bird or of the mammal^we find a series 

 of stages which ar$ ip. a general way parallel 

 ta. the historical evolution, of the heart. n.s we 



gra.Hps 



fish, amhibian and ^reptile. The same 

 T5e gained from a study of 



the development of the brain, the skull, the 

 kidneys, and other organs. It seems to us 

 impossible to deny that there is in the stages 



