To T. H. Huxley   13 [March 1859]¹

My dear Huxley

Very sincere thanks for your note, which tells me everything which I wanted.²

I am very glad to hear about the serially modified & homologous parts in the Radiata. Your sketch of one of the Diphydæ or Physophoridæ seems as good a case as any plant. I cannot remember, (though such is my memory I may have read it lately & have forgotten it) any remarks on the serial homologies of the Radiata; & I was astonished at this & hoped it might be that there was not much indefinite repetition of homologous parts, but how I forgot such a case as Tænia & some of the others you allude to, I cannot tell. In my ignorance I had looked at Echinus & Asterias as a definite repetition of one whorl of only 5 quasi-leaves, like the two halves of the body in annelids or Vertebrates &c.—

I have to allude to this subject, because the ideal morphologies of naturalists, are on my notions, real changes in the course of time from one part or organ into another.³

With many thanks | Yours very truly | C. Darwin

I entirely agree with your remarks on Agassiz’s Essay on Classification: it is all utterly impracticable rubbish, about his grades &c &c.⁴ But, alas, when you read, what I have written on this subject, you will be just as savage with me.