XIII] OF SHEEP AND GOATS 879 



is very nearly straight; in other species of the same genus OryXy 

 such as the Beisa antelope and the Leucoryx, a gentle curve (not 

 unlike though generally less than that of a Dentalium shell) is 

 evident; and the spiral angle, according to the few measurements 

 I have made, is found to measure from about >20° to nearly 40°. 

 In some of the large wild goats, such as the Scinde wild goat, we have 



Fig. 484. Head of Arabian wild goat, Capra sinaitica. 

 After Sclater, from P.Z.S. 



a beautiful logarithmic spiral, with a constant angle of rather less 

 than 70°; and we may easily arrange a series of forms, such for 

 example as the Siberian ibex, the moufflon, Ovis Ammon, etc., and 

 ending with the long-horned Highland ram: in which, as we pass 

 from one to anothei', we recognise precisely homologous spirals with 

 an increasing angular constant^ the spiral angle being, for instance, 

 about 75° or rather less in Ovis Ammon, and in the Highland ram 

 a very little more. We have already seen that in the neighbourhood 

 of 70° or 80° a small change of angle makes a marked difference in 



