My dear Sir.

I thank you sincerely for your most kind letter & I return your wishes for the new year with all my heart.¹ Your letter has interested me greatly. D^r Sanderson showed me some admirable photographs on glass by D^r Koch of the organisms which cause splenic fever; but your letter & the valuable work which you have kindly given me make the case much clearer to me.² I well remember saying to myself, between 20 & 30 years ago, that if ever the origin of any infectious disease could be proved, it would be the greatest triumph to science; & now I rejoice to have seen this triumph.

With respect to the filaments of Dipsacus, I do not for a minute put my judgement on a par with yours or that of de Bary, but my son has lately made some observations which incline me very strongly to believe that the filaments consist of living matter of the nature of protoplasm³   Hearing from D^r Sanderson that thymol has a fatal effect on low organisms, he tried solutions of $\frac{1}{10}$% & $\frac{1}{20}$% both of which cause contraction of the filaments. On the other hand $\frac{1}{2}$% solution of carbolic acid does not cause contraction, 1% does so, this agrees with several observers who find, I believe, that $\frac{1}{2}$% carbolic solution is not poisonous to microzymes. My son finds that strong solutions of NaCl cause contraction but not death, as the filaments recover themselves in water, & do not swell up into bladders as they do after poisons; this agrees with H. de Vries’ work on the ‘plasmolysis’ of cells—⁴

With cordial thanks & much respect | I remain, my dear Sir | Yours sincerely | Charles Darwin