126 REDUCTION OF VARIABILITY. 



police-dogs began, the Airedale was one of the first to be taken 

 up. It is now extensively used by policemen and as an ambu- 

 lance-dcg, and it is as well adapted for this special training as 

 dogs which wene especially bred as watch-dogs and police-dogs, 

 such as the French de Beauce, and the German Dobermann. 



Another police-dog of note is the German shepherd. This dog, 

 as well as the Collie and other sheep-dogs, such as the Dutch and 

 the Belgian and the Brie, was bred by shepherds, and it has 

 been used, and is still being used extensively, to help the shep- 

 herd manage the flock, guard it, separate animals of individual 

 owners. Young dogs learn the work directly from well-trained 

 older dogs. Hardly any dog is as well adapted to its special 

 work as these shepherd dogs. Now most of these dogs, espec- 

 ially the German and the French de Brie are being used as 

 police-dogs and they are as well adapted to this work as any 

 other species. That it has not the same faculties which are use- 

 ful both in sheep-driving and in the hunting of criminals is 

 shown by the remarkable fact that the Collie will make an 

 excellent sheep-dog, but is left severely alone by the policemen. 



The South-Dakota Experiment station is now breeding Per- 

 sian and South-Russian sheep, which were developed as fur- 

 bearers, but which, because of their fat tails or rumps are es- 

 pecially adapted to countries with a heavy snow-fall where they 

 have to starve occasionally. 



If we find two rather closely related species living in quite 

 different environment, and which show a difference which 

 makes them fit better each in his own mode of life, we do not 

 know whether this anatomical or physiological diffeience is 

 the result or the cause of this difference in habits or in environ- 

 ment. A good instance is that of the house-rat and tree-rat, both 

 sub-species of Mus rattus. The house-rat occurs in sheltered local- 

 ities, in houses, barns, ships, whereas the tree-rat lives more in 

 the open, nests in the axils of the leaves of palm-trees, forages 

 in the trees as well as in houses near trees. This latter rat will, 

 if pressed, swim and dive, and it will come up again practically 

 dry, like the Norway rat. In water the fur of the house-rat be- 



