THE BREAKING OF A SYNGAMIC CHAIN 89 



would yield results valuable in many ways. If inter- 

 breeding did not take place, or if the unions were sterile, 

 then we should have the interesting case of a single 

 species which would instantly become two if through any 

 circumstance a central link dropped out of the chain. 

 Even if chrysippus yielded negative evidence in this 

 respect, it is highly probable that other widely-distributed 

 species would, under these circumstances, fall into two 

 or more groups, each held together by interbreeding, and 

 divided from others by Asyngamy. 



Sterility, if present in any degree, would have been 

 brought about quite independently of selection ; for 

 in such cases each link of the chain would be freely 

 syngamic with the links on either side, and Asyngamy 

 or sterility would only be revealed by artificially bringing 

 together the widely-separated ends of the chain. 



I cannot but think, therefore, that such experiments 

 made upon many carefully-selected species would probably 

 bring important additional evidence to bear upon the 

 controversy as to whether sterility between species is, 

 as Wallace believes, a selected quality, or, as Darwin 

 held, an incidental one. The deep interest of this 

 question is realized when we thus remember that the two 

 discoverers of Natural Selection held widely different 

 opinions about it. We cannot read the letters on both 

 sides, printed in the first volume of More Letters, 

 without realizing how deeply this divergence — one of the 

 principal differences between them — was felt by the two 

 great naturalists. 



This is one of the many reasons for which I plead 

 with Mr. Roland Trimen 1 for the establishment of 

 tropical biological stations where work of the kind could 

 be carried on. Such establishments should be associated 

 with and be under the control of museums at home, 

 where the experiments could be directed and the results 

 studied and made available for all time for the researches 



1 In his two Presidential Addresses to the Entomological Society of 

 London. Proc. Enl. Soc. Lond., 1897, pp. xcvi, xcvii ; 1898, pp. lxxvii, 

 lxxviii. See also his remarks from the Chair in the discussion on May 5, 

 1897, loc. cit. pp. xxxi, xxxii. 



