82 VARIATION 



(March- Jul}^) the prawn appears to fill his burrow, possibly as 

 a provision against stormy weather, with large quantities of 

 minced seaweed, underneath which immense numbers of very 

 young EpMppodonta are found living.^ The extreme flatness 

 of the Ephippodonta must be due to the same cause as the flat- 

 ness of the Lepton noticed above, namely, the necessity of not 

 impeding or interfering with the lively motions of the prawn. 

 In the case of Lepton the two valves close completely and the 

 shell is still very flat ; in Ephippodonta^ on the other hand, the 

 same result is produced by the valves being opened to their 

 widest possible extent. As in Entovalva^ a continuation of the 

 mantle covers the outer surface of the shell. 



Variation 



It is a familiar experience to the student, not only of the 

 Mollusca, but of every branch of animal or vegetable life, to 

 come across examples which exhibit certain slight deviations 

 from the type form as usually understood. These deviations 

 may be more or less pronounced, but, as a rule, a series of forms 

 can be discovered, gradually leading up to or down from the 

 type. The definition of what constitutes a species, — and, still 

 more, the rigid application of such definition — will always 

 remain a diflicult task, so long as the personal element persists 

 in him who defines.^ What seems to one authority ample 

 ground for distinction of species, another may regard as of 

 comparatively trivial importance. The practical outcome of 

 these divergent views is sufliciently illustrated by the attitude 

 of Mr. F. P. Marrat on the one hand, and of what may be called 

 the modern French school of conchologists on the other. Mr. 

 Marrat holds, or held, that the great genus JVassa, of which more 

 than 150 species are generally recognised, is one shell (species) 

 in an endless variety of forms. The modern French school go 

 to the other extreme, and apparently proceed upon the view that 

 almost any difference in form, however slight, is sufiicient to 

 constitute a separate species. 



It will be generally admitted, however, that some structural 



1 E. H, Matthews, Conchologist, ii. p. 144. 



2 Thus Limnaea invohita, which is ahnost universally regarded as a good and 

 distinct species, has been held to be no more than a variety of L. peregra pro- 

 duced by locality j see Zoologist, 1889, p. 154. 



