364 K VOLUTION [Chap.V 



Lettei 275 I send by this post an essay by Hackel ' attacking Pan. 

 and substituting a molecular hypothesis. If I understand 

 his views rightly, he would say that with a bird which 

 strengthened its wings by use, the formative protoplasm of 

 the strengthened parts became changed, and its molecular 

 vibrations consequently changed, and that these vibrations 

 are transmitted throughout the whole frame of the bird, 

 and affect the sexual elements in such a manner that the 

 wings of the offspring are developed in a like strengthened 

 manner. I imagine he would say, in cases like those of Lord 

 Morton's mare, 2 that the vibrations from the protoplasm, or 

 " plasson," of the seminal fluid of the zebra set plasson vibrat- 

 ing in the mare ; and that these vibrations continued until 

 the hair of the second colt was formed, and which consequently 

 became barred like that of a zebra. I low he explains re- 

 version to a remote ancestor, I know not. Perhaps I have 

 misunderstood him, though I have skimmed the whole with 

 some care. He lays much stress on inheritance being a form 

 of unconscious memory, but how far this is part of his mole- 

 cular vibration, I do not understand. His views make nothing 

 clearer to me ; but this may be my fault. No one, I presume, 

 would doubt about molecular movements of some kind. His 

 essay is clever and striking. If you read it (but you must not 

 on my account), I should much like to hear your judgment, 

 and you can return it at any time. The blue lines are 

 Hackcl's to call my attention. 



We have come here for rest for me, which I have much 

 needed ; and shall remain here for about ten days more, and 

 then home to work, which is my sole pleasure in life. I hope 

 your splendid Medusa work and your experiments on pan- 

 genesis are going on well. I heard from my son Frank 

 yesterday that he was feverish with a cold, and could not 

 dine with the physiologists, which I am very sorry for, as I 



1 Die Perigenesis der Plasiidule odcr die J I 'ellenzeugung der Lcbcns- 

 theilcken, 79 pp. Berlin, 1876. 



-' A nearly pure-bred Arabian chestnut mare bore a hybrid to a 

 quagga, and subsequently produced two striped colts by a black Arabian 

 horse : see Animals and Plants, I., p. 403. The case was originally 

 described in the Philosophical Transactions, 1821, p. 20. For an account 

 of recent work bearing on this question, see article on "Zebras, Horses, 

 and Hybrids," in the Quarterly Review, October 1S99. See Letter 235. 



