82 VARIATION 



July) 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 

 Uphippodonta are found living.^ The extreme flatness of the 

 E])hippodonta must be due to the same cause as the flatness 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 sm^face 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, gradvially 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 difficult 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 sufficiently 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 Nassa, 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 sufficient to con- 

 stitute a separate species. 



It will be generally admitted, however, that some structured 



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



^ Thus 'Limnaea involuta, ■which is almost universally regarded as a good and 

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

 by locality ; see Zoologist, 1889, p. 154. 



