474 Recent freshwater Molluscaj 



ing the assumption to the contrary, by authors who have little 

 practical acquaintance with the details of natural history,* the 

 proper discrimination between species and variety is one of 

 the greatest difficulties which the naturalist has to encounter ; 

 and he who is successful in this department is entitled to a 

 rank which comparatively (ew can attain.f 



In the second place, although we may not be able, artifi- 

 cially, to produce a change beyond a definite point, it would 

 be a hasty inference, to suppose that a physical agent, acting 

 gradually for ages, could not carry the variation a step or two 

 farther; so that, instead of the original, we will say four va- 

 rieties, they might amount to six, the sixth being sufficiently 

 unlike the earlier ones to induce a naturalist to consider it 

 distinct.! It will now have reached the limit of its ability 

 to exist as the former species, and must be ready either to 

 develop a dormant organic element, or die ; if the former is 

 effected, the osculating point is passed, and the species estab- 

 lished upon the few individuals that were able to survive the 

 shock. If the physical revolution supposed to be going for- 

 ward is arrested or recedes, the individuals which had not 

 passed the culminating point remain as a fifth variety, or re- 

 lapse towards their former station ; whilst the few which have 

 crossed the barrier remain permanently beyond it, even under 

 a partial retrogression of the causes to which they owed their 

 newly-developed organization. We may suppose some spe- 



* This remark does not apply to Mr. Lyell, whose arguments indicate a knowl- 

 edge of the subject seldom apparent iu the writings of those to whom I allude 

 generally. 



t " In very extensive genera, the distinctions of species are so minute, that it re- 

 quires the most practised eye to separate them ; and, indeed, there are some groups, 

 the species of which are so intricately blended together, that no two entomologists 

 are agreed as to their distinctness." — Westwood. 



t This slight organic change might bring it sufficiently near a cognate species to 

 allow of the prodviction of a prolific hybrid, previously impossible; for, as Mr. Lyell 

 remarks, (Principles, ii. 372, London, 1837,) " Hybrids have sometimes proved pro- 

 lific, where the disparity was not too great." He asks, (p. 434) "if species in gen- 

 eral are of hybrid origin, where are the stocks which combine in themselves the 

 habits, properties, and organs, of which all the intervening species ought to afford 

 us mere, modifications?" An answer may be partly found in the complaint of 

 Professor Henslow, (Mag. Zool, Bot. i. 117,) that botanists describe certain species 

 as "duabus prioribus exacti intermedia." 



