THE VEGETABLE AND ANIMAL KINGDOMS. 115 



plete development, as knowing it to be not needed in that 

 instance. On this ground, the notion of a much ridiculed 

 philosopher of the last century, respecting a human tail, may 

 be said to be not quite without foundation. Between the fifth 

 and sixth week, a tail exists in the human embryo ; it then 

 goes back; but still in the mature subject its elements are 

 seen clumped up in the bone at the bottom of the spine, the 

 os coccygis. 



Unity of organization becomes the more remarkable when 

 we observe that the corresponding organs of animals, while 

 preserving a resemblance, are sometimes put to different uses. 

 For example, the ribs become, in the serpent, organs of loco- 

 motion, and the snout is extended, in the elephant, into an 

 instrument serving all the usual purposes of an arm and 

 hand. 



It is- equally remarkable, that there should be, in the ori- 

 ginal plan of the animal structure, a double set of organs, one 

 or other of which is selected for development according to the 

 needs of the particular animal. Thus, there are in the plan 

 both gills and lungs, two wholly distinct kinds of respiratory 

 apparatus, the one being designed for a watery and the other 

 for an atmospheric medium. The mammalia, as creatures 

 destined to breathe the air, are furnished with lungs ; but, at 

 an early stage of the foetal progress, this is not the case. 

 They have at that time a branchial apparatus. Afterwards, 

 this goes back, and the lungs are developed from a different 

 portion of the organism. Lungs, on the other hand, are 

 possessed by certain fishes in a rudimental form ; it is the 

 well-known air-bladder of those fishes, which are understood 

 to profit by it, as an additional means of floating. So, also, 

 the baleen of the whale and the teeth of the land mammifer 

 are different organs. The whale, in embryo,' shows the rudi- 

 ments of teeth ; but, not being wanted, they are not deve- 

 loped, and baleen is brought forward instead. 



But the most remarkable circumstance attending the law 

 of unity of organization is, that an organ will sometimes be 

 seen developed to a certain extent, but wholly without use. 

 i2 



