My dear Sir

I am very much obliged for your letter which has gratified me extremely, as I value your good opinion on the subject of this book, more than that of almost any other man in Europe.¹

I assumed that the matter which becomes aggregated was protoplasm, as I did not believe that any other known substance had the power of such peculiar amœboid movements. You will find that I describe the protoplasm that flows round the walls to be colourless, and that the granules which it contains are ultimately united with the central masses.² After the process of aggregation these masses float in colourless or almost colourless fluid, and I assumed that the colouring matter of the fluid was filtered or sifted out of it by the protoplasm, as it coalesced or aggregated. Do not the grains of chlorophyll consist of protoplasm coloured by this matter; & why should not the aggregated masses be tinted in the same way by the red colouring matter? But you will know all this far better than I do.

Pray again accept my thanks & believe me Dear Sir | Yours sincerely | Charles Darwin