In certain instances differences in the morphology of genitalia between species make it impossible for normal mating to occur, and sympatric species tend to remain isolated. When there is no correspondence between the male and female external genitalia for copulation to occur, the members belonging to different species are reprod;ctively isolated. When interspecific crosses occur between individuals having no exact correspondence in their genitalia, it resulted in the death of copulating pairs. Isects and snails are usually quoted as examples of such fatal ratings which occur "due to mechanical differences in their genitalia. It must alao be stated that differences in cogulatory organs, in many instances, have not proved to be a barrier for ihterbreeding. Breeding between dogs belonging to different races is an often cited example.

Mechanical isolation appears to play a more important role in the speciation of plants. Since many plants are aided by insects and birds in cross pollination, a morphological compatibility is required between the plants and the pollinating agents. Further, queen bees with their long mouth parts pollinate the nector producing species of Pedicularis and the other species of the plants are pollinated sternotribically or nototribically.