EVOLUTION OF THE HUMAN SPECIES 171 



inary conditions through which the human organism 

 passes in development, we are forcibly impressed by 

 other facts than the one to which I have directed your 

 attention, for not only do we find natural transforma- 

 tion, as in the other mammals, but the embryonic stages 

 are marvelously similar to the earlier conditions in 

 other mammals. Not very long before birth the human 

 embryo is strikingly similar to the embryo of the ape ; 

 still earlier, it presents an appearance very like that of 

 the embryos of other mammals lower in the scale, like the 

 cat and the rabbit, forms which comparative anat- 

 omy independently holds to be more remote relatives 

 of the human species. Indeed, as we trace back the still 

 earlier history, more and more characters are found 

 which are the common properties of wider and wider 

 arrays of organisms, for at one time the embryo exhibits 

 gill-slits in the sides of its throat which in all essential re- 

 spects are just like those of the embryos of birds and 

 reptiles and amphibia, as well as of other embryo mam- 

 mals ; and these gill-slits are furthermore like those 

 of the fishes which use them throughout life. All the 

 other organic systems exhibit everywhere the common 

 characteristics in which the embryos of the so-called 

 higher animals agree with one another and with the 

 adult forms among lower creatures ; the human embryo 

 possesses a fishlike heart and brain and primitive back- 

 bone, fishlike muscles and alimentary tract. Can we 

 reasonably regard these resemblances as indications of 

 anything else but a conamunity of ancestry of the forms 

 that exhibit them? 



Yet a still more wonderful fact is revealed by the 

 study of the very earliest stages of individual develop- 



