THE ARGUMENTS FROM EMBRYOLOGY. 455 



regularity of sequence. In some cases they must have been 

 more numerous than in others; in some cases they must 

 have been greater in degree than in others; in some cases 

 they must have been to simpler modes, in some cases to 

 more complex modes, and in some cases to modes neither 

 higher nor lower. Of two cognate races which diverged in 

 the remote past, the one may have had descendants that 

 have remained tolerably constant in their habits, while the 

 other may have had descendants that have passed through 

 widely-aberrant modes of life; and yet some of these last 

 may have eventually taken to modes of life like those of the 

 other races derived from the same stock. And if the meta- 

 morphoses of embryos indicate, in a general way, the changes 

 of structure undergone by ancestors; then, the later embryo- 

 logic changes of such two allied races will be somewhat 

 different, though they may end in very similar forms. An 

 illustration will make this clear. Mr. Darwin says: 

 " Petrels are the most aerial and oceanic of birds, but in the 

 quiet sounds of Tierra del Fuego, the Puffinuria herardi, in 

 its general habits, in its astonishing power of diving, its 

 manner of swimming, and of flying when unwillingly it takes 

 flight, would be mistaken by any one for an auk or grebe; 

 nevertheless, it is essentially a petrel, but with many parts of 

 its organization profoundly modified." Now if we suppose 

 these grebe-like habits to be continued through a long epoch, 

 the petrel-form to be still more obscured, and the approxi- 

 mation to the grebe-form still closer; it is manifest that 

 while the chicks of the grebe and the Puffinuria will, during 

 their early stages of development, display that likeness in- 

 volved by their common derivation from some early type of 

 bird, the chick of the Puffinuria will eventually begin to 

 show deviations, representative of the ancestral petrel-struc- 

 ture, and will afterwards begin to lose these distinctions and 

 assume the grebe-structure. 



Hence, remembering the perpetual intrusions of organisms 



on one another's modes of life, often widely different; and 

 30 



