296 CAUSES OF VARIATION 



SECTION I EVIDENCE FROM STABILITY OF TYPE 



Although no two individuals are alike, and although a given 

 character differs greatly in different members of the race, yet 

 there is a specific type that is always and everywhere present ; 

 though the elements are exceedingly variable, yet the resultant 

 composite is remarkably constant as compared with other types. 



In other words, when we compare horses with horses we are 

 impressed with the fact of variability, but when we compare 

 horses with cattle, or even with asses, then we are led to marvel 

 at the fixity and persistence of type. In all its variations, the 

 horse is still clearly a horse. Wide and profound as is variability, 

 it is yet well .within limits, and certain types continue with 

 singular persistence. 



The Hubbard squash and the Morgan horse are good exam- 

 ples of persistence of type. With all the variability of the Cu- 

 curbitaceae, the Hubbard squash persists, distinct in type and 

 quality. It mixes freely with other types, -but its characters are 

 evident even then, and it possesses a singular ability to free itself 

 from such admixtures and return again to the original. 



The Morgan horse is a breed established by a single animal, 

 and yet, a hundred years after the death of Justin Morgan, when 

 the per cent of his blood is of necessity slight, the Morgan 

 characters still stand out clearly, constituting a type almost as 

 distinct as that of any existing breed. This is the solitary known 

 instance of the founding of a breed by a single ancestor. It 

 illustrates in a peculiar way the occasional extreme persistence 

 of a type once formed, and is in marked contrast to the readiness 

 with which other types break up and disappear. 



The old-time persistence of the sloping rump in the Berkshire, 

 of the narrow chest in the Poland-China, of lack of depth behind 

 in the pony-built Hereford, of deficient crops in the Shorthorn, 

 these and similar defects that might be mentioned illustrate 

 the strength with which certain characters continue, even in the 

 face of the most powerful opposition, and argue strongly for 

 stability of type. 



It is also a general fact that species hold their types with 

 essential success under a great variety of conditions, both 



