JTJLT 17, 1914] 



SCIENCE 



99 



^uite so large a moutli could be utilized if the 

 pipette were to be used in litjuids lighter than 

 water.) It is a most useful collecting con- 

 venience. In places where a small net could 

 not be used because of stones or other debris 

 and in handling objects liable to injury in a 

 net the writer has found it almost indispen- 

 sible. The capacity of the rubber bulb and the 

 large mouth of the pipette make it possible 

 by means of a sudden suction to catch small 

 animals too quick to be taken in a small gauze 

 net. The glass bulb retains all the sucked-in 

 water, enabling one to see what has been taken 

 and obviating the difficulty of losing or mu- 

 tilating a choice specimen by getting it into 

 the rubber bulb. The pipette is not over- 

 fragile and is short and convenient for slip- 

 ping into the pocket or collecting case. It is 

 also more convenient to use than the long 

 pipette in common use. 



The calcium chloride tubes are made in vari- 

 ous sizes. For a smaller pipette a rubber bulb 

 of 25 c.c. capacity and a calcium chloride tube 

 150 mm. long may be used. The cost of the 

 pipette is slight. 



Arthur M. Banta 



Station tor Expeeimental Evolution 



is melanism due to foodf 

 It is a well-known fact that occasional dark- 

 colored individuals occur among wild animals 

 of various kinds. Once in a while a pure black 

 beaver is caught. Pur traders sometimes pick 

 up skins of mink, otter, marten and other ani- 

 mals which are coal black. These skins are 

 especially valuable. Perhaps the best known 

 instance of dark specimens occurring in a spe- 

 cies ordinarily light in color is that of the 

 silver or black fox, which may be one in a lit- 

 ter of common red foxes. 



This occurrence of dark animals is called 

 " melanism," but so far science has failed to 

 ascertain the cause. It is my purpose in this 

 article to call attention to some facts which 

 may or may not throw some light on the sub- 

 ject, but which seem to me to be at least sug- 

 gestive. 



Northern Minnesota has been a great fur- 

 producing region ever since the Hudson Bay 



Company established posts here at the head- 

 waters of the St. Lawrence, the Mississippi and 

 the Eed Eiver of the North. "We should ex- 

 pect, and the fur traders actually get choice 

 furs from this cold, high, heavily wooded land 

 of lakes. The high grade of furs obtained in 

 northern Minnesota is well known to the trade; 

 the value of the annual catch, over a million 

 dollars, is something less widely known. 



It is commonly supposed that the relative 

 proportion of different kinds of fur caught in 

 the state runs along fairly constant year after 

 year. This is not the case for two reasons. A 

 series of years may be favorable for the increase 

 of a species, resulting for a time in an abnor- 

 mally heavy catch of that animal. As an in- 

 stance of this, we cite the Canadian lynx, 

 which increases with the abundance of the 

 snowshoe rabbit, and suffers or migrates at in- 

 tervals when its food supply has been seriously 

 reduced by the dying off of the rabbits from the 

 so-called " rabbit plague." Perhaps a better 

 instance is that of the muskrat, which may in- 

 crease because of several winters during which 

 ice and water conditions are favorable to its 

 " wintering over." The other reason is that 

 which gives rise to this article. A species like 

 the red fox may suddenly show an unusually 

 strong tendency to vary from its type. 



Ordinarily there are caught annually in 

 northern Minnesota somewhere in the neigh- 

 borhood of fifteen hundred red foxes. Of this 

 number of skins, we venture to guess that, for 

 the five years preceding the winters of 1911 and 

 1912, not more than ten each year were sold 

 as black or silver foxes, and not over forty as 

 cross foxes. The wiater of 1911-12 saw a 

 marked increase in the number of high-grade 

 fox skins brought in to the posts, and there 

 was a still further increase in 1912-13. In 

 the Eainy River watershed, especially, it 

 seemed as if about one fifth of the foxes 

 caught last winter were either dark, silver or 

 cross foxes. This winter, 1913-14, the per- 

 centage of these high-grade color phases is 

 even higher. 



During the past three years there has been 

 an abundance, amounting almost to a plague, 

 of mice (white-footed wood-mice) in the 



