I56 'DESCENT OF MAN' — EXPRESSION. [1872. 



chologist to investigate, viz. : whether the species of the same 

 genus were variable during many successive geological forma- 

 tions. I began to make enquiries on this head, but failed in 

 this, as in so many other things, from the want of time and 

 strength. In your remarks on crossing, you do not, as it 

 seems to me, lay nearly stress enough on the increased vigour 

 of the offspring derived from parents which have been exposed 

 to different conditions. I have during the last five years 

 been making experiments on this subject with plants, and 

 have been astonished at the results, which have not yet been 

 published. 



In the first part of your essay, I thought that you wasted 

 (to use an English expression) too much powder and shot on 

 M. Wagner ; * but I changed my opinion when I saw how 

 admirably you treated the whole case, and how well you 

 used the facts about the Planorbis. I wish I had studied 

 this latter case more carefully. The manner in which, as 

 you show, the different varieties blend together and make 

 a constant whole, agrees perfectly with my hypothetical 

 illustrations. 



Many years ago the late E. Forbes described three closely 

 consecutive beds in a secondary formation, each with repre- 

 sentative forms of the same fresh-water shells : the case is 

 evidently analogous with that of Hilgendorf,| but the interest- 

 ing connecting varieties or links were here absent. I rejoice 

 to think that I formerly said as emphatically as I could, that 

 neither isolation nor time by themselves do anything for the 

 modification of species. Hardly anything in your essay has 

 pleased me so much personally, as to find that you believe to 

 a certain extent in sexual selection. As far as I can judge, 



* Prof. Wagner has written two tothe Bavarian Academy of Sciences 



essays on the same subject. ' Die at Munich, 1870. 



Darwin'sche Theorie und das f " Ueber Pla?iorbis multiformis 



Migrationsgesetz,' in 1868, and im Steinheimer Siisswasser-kalk." 



1 Ueber den Einfluss der Geogra- ' Monatsbericht ' of the Berlin Aca- 



phischen Isolirung, &c.', an address demy, 1866. 



