Responds to CD’s query about the blind fauna of Mammoth Cave.

Gives information from L. Agassiz. Distribution of Crustacea, especially along southern coastlines.

Agassiz has informed him that the mice and rats of Mammoth Cave are American in type.

Alludes to CD’s doubt of the principle that "progress of life on the globe is parallel with the development in different tribes". Outlines his own ideas on the "unfolding of the type-idea" and its "parallelism with the law of development in the embryo".