ISOLATION AND SEXUAL SELECTION. 83 



Closely allied rival forms often meet or overlap in some 

 region, yet interbreed little, or not at all, although quite 

 capable of forming a fertile union and of producing 

 fertile offspring. A familiar instance is that of the 

 negroes and whites in America. Sterility in one form or 

 another is the most important physiological barrier, and 

 may be due to many different causes. For instance, 

 sterility may result from variation in the structure of 

 the copulatory organs, as in numberless kinds of insects ; 

 or merely in their size or shape. Intercrossing may also 

 be prevented if sexual maturity is reached at different 

 times of the year ; and self -fertilisation is made impossible 

 for most hermaphrodites by the spermatozoa developing 

 either before or after the ova in the same individual. 

 Lastly, isolation may result from some variation in the 

 germ-cells themselves, causing fertilisation to be imper- 

 fect or sterile even if it take place ; this may be called 

 true sterility. Further, the zygotes when formed may 

 fail to develop normally, or even if viable the hybrid 

 offspring may themselves be sterile. This form of ste- 

 rility is common among plants and animals ; the mule 

 produced from a cross between the horse and the donkey 

 is the most familiar instance. In theae various ways, 

 then, divergences inevitably arise among groups of 

 organisms originally alike, and evolution in its course 

 moves along ever branching paths. 



If a higher organism is to succeed in the struggle for 

 existence it must reproduce itself sexually ; hence the 

 importance in evolution of the various adaptations for 

 securing the fertile union of the sexes. The differences 

 between the two sexes, other than those of the repro- 

 ductive organs themselves, are known as secondary sexual 

 characters. Many are of such a kind as to enable them 

 to find and recognise each other, and to accomplish the 

 act of copulation. To this class belong the diverse 

 organs developed in all sorts of animals for grasping the 

 female, the call notes of many insects, birds, and mammals, 



