DIVIDING FACTORS. ISOLATION 437 



structed on the lock-and-key principle, only a certain pattern of key 

 fitting a certain pattern of lock. Any change in the pattern of these 

 organs prevents the members of the changed type from breeding with 

 the unchanged individuals and forces them to breed only with those 

 that have changed in appropriate fashion. Fortunately, when a 

 change occurs, it usually affects both sexes in such a manner that males 

 and females of the changed type can mate. No actual observations 

 of new races arising in this way have been observed, but there are 

 numerous cases of closely allied species that are intersterile because of 

 slight differences in the pattern of their copulatory organs. 



One of the most effective means of bringing about reproductive 

 isolation is a change in the developmental rhythm of a section of a 

 species. If, for example, some plants of a species flower earlier or later 

 in the season than others, they are able to interbreed only with those 

 having the same developmental rate. If such a difference in develop- 

 mental rate be genetic, a real isolation of these different races will be 

 effected and further divergence will follow. A similar situation also 

 occurs among animals. The genus Cicada, commonly called "locusts," 

 has probably split up into numerous species as the result of genetic 

 modifications in the lengths of their life-cycles. Whereas there are 

 only slight morphological differences among the species of Cicada, 

 there are pronounced differences in the lengths of the larval periods. 

 In the "seventeen-year locusts," for example, the larval life under- 

 ground is over sixteen years. As a result of this, only once in seventeen 

 years do the adult broods appear above ground. They must, there- 

 fore, mate only with members of their own brood in the same territory. 

 Similarly, there are fourteen-year, eleven-year, nine-year, and seven- 

 year species, etc., each of which is reproductively isolated in any 

 territory from all others, for rarely do two species reach maturity in 

 any region during the same season. Even in one species scattered 

 over a wide expanse of territory, different broods appear one or two 

 seasons apart, and this tends to isolate sections of the present species 

 from one another and will doubtless produce further splitting-up of 

 species as time goes on. Doubtless many other situations similar to 

 these, but less extreme, exist among both animals and plants. That 

 changes in developmental rate do exist within a species and have a 

 genetic basis, is attested by the fact that many early and late fruiting 

 varieties of domestic plants have been isolated, some of which are 

 more suitable for a long southern growing season and others for the 

 relativelv short northern sea 



