THE STUDY OF SHORE ANIMALS. 25 



tortoise-shell limpet does not fit nearly so closely to the 

 rock, its shell is much thinner, and its tissues more delicate ; 

 it is probably for these reasons that it never leaves the 

 pools. It must, of course, be realised that both are true 

 aquatic animals, and that a certain amount of moisture is 

 an essential of existence to both. The difference between 

 the power of adhesion of the two forms is so marked that 

 it can be employed as a means of distinguishing them where, 

 from depth of water or other cause, the characters of the 

 shell cannot be clearly seen. As everyone knows, the com- 

 mon limpet may be dislodged by a sudden and unexpected 

 blow ; but if the first attempt fail, the alarmed animal 

 adheres so tightly that a knife is necessary to detach it. 

 The tortoise-shell limpet, on the other hand, can always be 

 removed with the fingers alone. It never reaches the size 

 which the common limpet does, but in specimens of the 

 two forms of the same size the difference in the clinging 

 power is quite distinct. 



This description should be sufficient to permit of an easy 

 recognition of the two forms, and they should be studied 

 until eye, touch, and muscular sense are so trained that there 

 is no possibility of error. This may seem a trivial occupa- 

 tion, but some preliminary training of this kind is essential 

 to anyone desirous of acquiring an acquaintanceship with 

 species; and the identifying of species, though now sadly 

 out of fashion, is an occupation which may yield one of the 

 subtlest of pleasures. Of late years so much has been said 

 of variation and its consequences, that not only the general 

 public, but even some zoological students, seem to have an 

 idea that species were something abolished by Darwin, and 

 that the notion that there is constancy and orderliness in 

 nature is a mediaeval myth. It may well be that the older 

 naturalists made too much of that constancy, and toiled 

 over their species-mongering until they reduced the organic 

 world to the condition of a labelled liortus siccus instead of 

 a living, growing reality ; but it does not appear that our 

 gain is great if we swing to the opposite extreme, and 

 inculcate the idea that there is no constancy or definiteness 

 in nature at all. So much of the present-day academical 

 teaching seems to have this result, that I cannot but urge 

 anyone beginning open-air studies to find some time for 



