From J. S. Burdon Sanderson   16 October [1874]

Dear M^r Darwin,

I do not think that it is possible to regard areolar tissue or the fibrous basis of bone as allied to gelatine.¹ They are often called collagens & said to consist of gelatigenous substance, but all that this means is that they yield gelatine when boiled in water. In the natural state they do not contain any gelatine.

Fibrocartilage, fibrous basis of bone and areolar tissue are closely allied, but there is no relation between them and fibrin, which is a genuine albuminous substance.

I still think that the common property by which the 5 substances are linked together, must be their not containing nitrogenous material soluble in water.²

If it were possible for me to examine the substances actually used, I should be able to ascertain whether this is so or not.

If in this way or any other I can be of the slightest service, it will afford me the greatest pleasure

I am, Dear M^r Darwin | very truly yours | J B Sanderson