II.] INTRODUCTION. XXvii 



union between Helix nemoralis and H. hortensiSy in which 

 the colour of the lips of their shells in each case varied 

 indifferently from bro^Ti or rose-colour to milk-white. 

 Professor Lecoq and M. Miergue have celebrated the 

 voluntary nuptials between individuals of Helix nemo- 

 ralis and H. aspersa, as well as between Pupa cinerea 

 and Clausilia papillaris ; but these unions do not seem 

 to have been blessed with any offspring. The fact, 

 however^ of such unions having taken place in a state of 

 nature, and not under forced or accidental conditions, 

 is remarkable, and the more so because the individuals 

 belonged to what are considered different genera. 



Progressive development. — The researches of geolo- 

 gists have established by positive evidence,, that the 

 organization of many animal and vegetable types has not 

 become more specialized or rendered more perfect since 

 the period to which we ascribe their creation, and that, 

 notwithstanding the enormous lapse of time which is 

 indicated by the accumulation of fossiliferous strata, the 

 modification or change which these types have under- 

 gone has been remarkably slight. There is abundant 

 evidence of variation, but none of what is usually under- 

 stood as progression*. The theory of ^'^progressive de- 

 velopment '^ appears to have been very hastily advanced, 

 and is by no means borne out by geological facts. It is 

 a cmious circumstance in the history of the growth of 

 certain land and freshwater Mollusca, that the young of 

 some species of Pupa resemble those of Helix, the young 

 of the latter those of Zonites, and the young of the last 

 those of Vitrina, These genera are enumerated in the 

 order of their organization, Pupa being the most, and 

 Vitrina the least perfect of them. 



* See Professor Huxley's Address delivered at the Anniversary Meeting 

 of the Geological Society, 21st February, 1862. 



62 



