RELATIONSHIP OF SPECIES. 95 



raent, habit, and environment might be expected to very greatly 

 affect the other organs, there would appear no very great reason 

 why the form of the former should necessarily change, nor, 

 broadly speaking, do they. Copulative organs and actions can 

 have apparently no great reason for changing, nor would one 

 expect them to be so liable to be influenced by evolution as 

 bodily form, and they certainly appear to be the last to change ; 

 and in this respect the divergence in these organs between 

 Platyrrhine and Catarrhine primates appears to be extremely 

 suggestive of long separation and great divergence. 



One feels bound to refer to divergence of results as shown in 

 some reciprocal crosses between the same two species. Darwin 

 refers to Kolreuter's experiments (' Origin of Species ') thus : — 



" Miribilis jalapa can easily be fertilised by the pollen of 

 M. longiflora, and the hybrids thus produced are sufficiently 

 fertile, but Kolreuter tried more than two hundred times, during 

 eight following years, to fertilise reciprocally M. longiflora with 

 the pollen of M. jalapa, and utterly failed." 



An explanation of this curious fact appears hard to find. As, 

 however, there does not appear to be any difference in the 

 fertility of the hybrids that can be bred reciprocally, possibly 

 some theory of " uncongeniality " might apply to these cases. 

 When definite light is shed on such problems, one may expect 

 to see much of the puzzle of hybridism laid bare. Considering 

 the fact that many females are not fertile with certain males of 

 their own species, it seems unwise to lay too much stress on the 

 aberrant infertility of these reciprocal combinations. 



If the theory of gradually separating forms shown by 

 infertility is correct, some intermediate form should connect two 

 others. No definite experiments to test this have been made, as 

 far as I know. 



Kolreuter, however, experimented with five forms or varieties 

 of the common tobacco, and found that, although all these 

 forms and their hybrids were perfectly fertile (even when tried 

 reciprocally), one of these forms when crossed with Nicotiana 

 glutinosa produced hybrids less sterile than the other four forms 

 produced when crossed with this species. 



After summarising the examples given above, and in the 

 endeavour to draw deductions from them, one is prepared to 



