(C44) 
with the indigenous forms of chrysippus ? 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 chrysippus 
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 syngamie 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, where 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, 
