( cxv ) 



tioTi, it is found tliat tliese differ very considerably in structure 

 in closely allied species. It has been remarked that they 

 oft'er some of the best characters to distinguish species, and 

 they have been made use of to separate species which scarcely 

 offered any other distinguishable characters. Mr. Baly has 

 also discovered that the horny penis concealed in the male 

 abdomen of Phytophagous Coleoptera differs in form in closely 

 allied species; and he has shown me a long series of specimens 

 mounted for examination under the microscope, belonging 

 chiefly to the genera Chrysomela and Eumolpus, which offer 

 a most instructive study, since by their means some forms 

 before considered as varieties turn out to be distinct species. 



" This class of facts seems to me of great significance, as 

 throwing light on the segregation of varieties and their jjassage 

 into true species. For if we admit that the only soimd differ- 

 ence between allied varieties and allied species is that the 

 former intermarry and the latter do not, then the abrupt 

 and great diversities of structure in those organs most directly 

 involved in the matter must be considered as affording an 

 explanation why many varieties do not intercross with the 

 parent stock and therefore remain as independent forms or 

 species. The difference in the accessory male organs of our 

 two allied species or local forms of Colohoihea is so great that 

 no one who examines them can believe both to be adapted 

 to the corresponding organs of the females of each form. At 

 the same time I have no doubt that, were it not for the great 

 difference between these organs in our two forms, no entomo- 

 logist would doubt their being mere local varieties of one and 

 the same stock. .Scores of other local varieties occur in the 

 same countries, presenting all the successive steps of segre- 

 gation, from the most partial variation to the full-formed 

 local race. 



" Thus we have only to admit that species disseminate them- 

 selves over wide areas, and adjust tJiemselves to the diversities 

 of local conditions, or, in other words, segregate local varieties, 

 to open the way towards an explanation of the way in which 

 the world has become peopled by its myriads of species. The 

 inevitable law of Natural Selection which governs the general 

 process of the adjustment of the local races to new conditions 



