My dear Mr Darwin—

I have received from Professor Agassiz, thro’ Dr Gray, the reply that the mice or rats of the Mammoth Cave are American in type, as to teeth &c.— ¹

You say you doubt the principle that the progress of life on the globe is parallel with the development in different tribes. I would not accept of it to the same extent as Agassiz. Yet although far from universal, it appears now & then to be exhibited, as in the case of the Ganoids mentioned in my former note.—² The principle is subordinate to a more general law of progress—a law which involves the expression of a type-idea in forms or groups of increasing diversity, and generally of higher elevation; always resulting in a purer & fuller exhibition of the type. There may be an expansion to lower as well as higher groups, as the development of an embryo brings out the [illeg] of the surface as well as the brain & viscera. But in all cases, it is a more & more complete exhibition or unfolding of the type-idea, until its maximum display is made.

It is the comprehensive before a multiplicity of specialities, and under this restriction it is the simple before the Complex.— I cannot but read this much at least from our knowledge of the life of the globe, and see so far a parallelism with the law of development in the embryo, which brings multiplied details, beginning with those of most comprehensive signification, from the memberless unit. Under such a principle, it is natural that the history of an individual in its particulars should sometimes run parallel with that of the palæontological history of the tribe to which it pertains. But the fundamental parallelism is in the grand law alluded to, not in these occasional lines under it. Looking at the progress of life from this point of view, I should find no principle upset by the discovery of barnacles far lower in the series of rocks than they now are known to occur.³ The details under the law are yet to be fully made out; and when so, I believe the law will appear in far bolder Characters that now are manifest.

Excuse my homily on this Subject, and believe me always glad to aid you in any way within my power; for I believe there is real truth in the results of your labors, and the best of foundations for general laws or principles.

very truly yours | James D. Dana