THE STABILITY OF SPECIES 41 



terest; for the subject is one which has hitherto at- 

 tracted but little attention. Since the Darwinian 

 hypothesis was given to the world we have heard so 

 much of variation and the origin of new species that 

 the other phenomenon that of the fixity of species in 

 spite of varying environments has been almost entirely 

 overlooked. Yet it was just this feature of animal life 

 that attracted the attention of the older zoologists and 

 led them to believe that species had been created once 

 and for all, and that, when created, they were immutably 

 fixed. 



Most biologists, if asked to explain the comparative 

 fixity of species, the slowness of evolution, would, I 

 think, refer to the fact that variations appear to take 

 place indiscriminately in all directions. Take, for 

 example, a large number of birds of any species and 

 measure any one organ, let us say the first primary 

 wing feather. Suppose the average length be six inches. 

 We shall find that in a considerable percentage of the 

 individuals measured the wing is exactly six inches in 

 length : that six inches is what we may call the favour- 

 ite or fashionable length of the wing. The next com- 

 monest lengths will be 5-99 and 6'Oi inches, and so on. 

 We shall find that only a very small percentage of the 

 individuals have wings shorter than 5j inches or longer 

 than 6 inches ; and if we measured a thousand in- 

 dividuals we probably should not find any in which the 

 wing was shorter than five inches or longer than seven. 



Now, the commonly accepted theory is that in those 

 cases where there is free interbreeding the long-winged 

 varieties and the short-winged varieties tend to neutralise 



