474 THE AMEBIC AN NATURALIST [Vol. XL VIII 



that we do not now see its processes in action, that species 

 are stable and subject to very little variation. This view, 

 however, was found to rest on faulty observation, for, 

 though some species are fairly stable, others are very 

 plastic and exhibit before our eyes various steps in 

 development. So in case of the development of a partic- 

 ular pied pattern, it is possible to see in certain species 

 the actual course of its formation. Among mammals, 

 the Mustelida* of weasel family show several instances in 



point. The common mink (Mustela vison) of north- 

 eastern North America is now in process of developing a 

 pure white under side, such as is present in the New York 

 weasel (M. noveboracensis) or the smaller Bonaparte's 

 weasel (M. cicognani). The diagrams shown in Figs. 

 43-48 are from the fine series of mink in the collection of 

 the Museum of Comparative Zoology and depicit the 



