46 EVOLUTION AND GENETICS 



That the visible structural differences between 

 groups of individuals is not a safe guide on which to 

 construct a real distinction, is exemplified by numer- 

 ous cases in which groups may exist side by side, that 

 are so closely similar that only an expert can sep- 

 arate them, yet are either infertile when crossed or 

 else leave offspring that are sterile. For example, 

 two species of Drosophila, often found together, are 

 so similar that complete separation is extraordi- 

 narily difficult. They can be made to cross only after 

 many matings, but the offspring are completely 

 sterile. With this information at hand no one would 

 hesitate to call them different species, while without 

 it the two would be placed in the same species. 



It is true, in general, that the more "different" 

 in structure two groups are, the smaller the chance 

 that they will be found to cross, and still smaller the 

 chance that they will have fertile offspring. This 

 brings us back to the second question concerning 

 the relation of infertility between species and the 

 sterility of species hybrids. The infertility may ap- 

 pear at first sight to be due to the observed differ- 

 ences, but there mav be some other less obvious 

 relation that is responsible for their failure to pro- 

 duce offspring. Darwin was familiar with this prob- 

 lem and has written about it extensively. He pointed 

 out that there are no sharp lines with respect to 

 infertility between species, and gave many exam- 



