HOMOLOGY. 77 



instance of homology is due to descent from a 

 common ancestor, but when the organs compared 

 are upon different individuals the common ancestor 

 was a remote independent animal, and when the 

 organs compared are upon the same individual the 

 common ancestor is a simple embryonic cell with 

 no appendages but with power to produce them. 

 The likeness between the man's arm and the bird's 

 wing is due to descent from an ancient intermediate 

 ancestor, that possessed an appendage with the 

 fundamental structure of both wing and arm. Man's 

 leg and arm owe their likeness to descent from a 

 certain cell, arising from the ovum, which has given 

 origin to both. 



This idea has been too little discussed to justify 

 any conclusion as to its adequacy to meet the ques- 

 tion, for indeed it has been hardly more than sug- 

 gested by one or two writers. There is some 

 embryological evidence in its favor, though as yet 

 not very extensive. It is certainly an internal con- 

 nection which many facts demand, and will enable 

 us to understand why the two arms and two legs 

 are liable to similar diseases, or why abnormalities 

 occurring in one would not be unlikely to occur in 

 all the others. Because any influence acting on the 

 embryonic cell would affect all appendages alike, it 

 would readily account for serial abnormalities of 

 such organs, of which numerous instances occur. 

 Whether it is sufficient to explain the exact homolo- 

 logies so noticeable in serial organs is not quite so 

 clear. But admitting this as a partial explanation, 

 we introduce a new factor into our theory of descent, 



