Variability and Inheritance. 137 



On the other hand certain descendants of the 

 reptile-bird emphasized the bird characteristics 

 and progressed away from the reptilian, until there 

 was developed the race of true birds. 



As birds and reptiles receded in structure from 

 each other they became strangers in more senses 

 than one, and lost all power of sexually attracting 

 each other. The reptile was aroused to expressions 

 of love by a reptile like itself, the bird by a bird like 

 itself, and it is owing to this loyalty of the repro- 

 ductive passion that species remain distinct. 



A robin loves only a robin, consequently the 

 robin race is steadfast. 



From the. lowest to the highest life, is every- 

 where seen this loyalty to kind. 



Reproduction is possible within but very narrow 

 limits; each form of life is isolated from every 

 other form, and even within the physical limits 

 there has arisen the limit of individual choice, so 

 that the robin will not accept any other robin as 

 a mate, but only such a one as it finds personally 

 agreeable. 



On the borderland of this physical reproductive 

 limitation creatures can be induced to accept each 

 other, but the result is contrary to nature, as is 

 shown in the sterility of the offspring. The mule, 

 for instance, is the descendant of a horse and an 

 ass, and often possesses no reproductive power, as 

 a penalty for coming from parents too far removed 

 from the bonds of perfect race sympathy. 



