( cxii ) 



with the indigenous forms of chryslpjnis ? We do not know ; 

 but it is an experiment well worth trying, and one which 

 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 chnjsipjms 

 yielded negative evidence in this respect, it is highly probable 

 that other widely-distributed species would, under these cir- 

 cumstances, fall into two or more groups, each held together 

 by inter-breeding, 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 realised when we thus re- 

 member 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 realising 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 for the establishment of tropical bio- 

 logical 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, wliere the experiments 

 could be directed and the results studied and made available 

 for all time for the researches of the naturalist. Just as 

 Harvard has her main Observatory at the University, but also 

 maintains an outlying institution in the Peruvian Andes, 

 where certain kinds of research, unsuited to New England, 



