1872.] ISOLATION. 335 



through "Amixie," is altogether new to me, and seems to 

 throw an important light on an obscure problem. There i'o, 

 however, something strange about the periods or endurance 

 of variability. I formerly endeavoured to investigate the 

 subject, not by looking to past time, but to species of the 

 same genus widely distributed ; and I found in many cases 

 that all the species, with perhaps one or two exceptions, were 

 variable. It would be a very interesting subject for a con- 

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

 genus were variable during many successive geological forma- 

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

 ihis, 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 con- 

 stant 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, f but the inter- 



* Prof. Wagner has written two essays on the same subject. ' Die Dar- 

 win'sche Theorie und das Migrationsgesetz, in 1868, and ' Ueber den Ein- 

 fluss der Geographischen Isolirung, &c.,' an address to the Bavarian Acad- 

 emy of Sciences at Munich, 1870. 



f " Ueber Planorbis multifotmis im Steinheimer Siisswasser-kalk/ 

 Monatsbericht of the Berlin Academy, 1866. 



