SOME STRANGE MARRIAGE-CUSTOMS 273 



quisite beauty in the form and sculpturing of the shell 

 which so many species exhibit, are characters which 

 cannot be regarded as due to sexual selection. 



As touching the danger of self-fertilization to which 

 reference has been made. That this is real is shown 

 by the fact that the ova and spermatozoa are rarely ripe 

 in one individual at the same time. However, among 

 the pulmonata, or air-breathing gastropods, it seems to 

 have been established that self-fertilization can, and 

 does, occur. That in some species, at any rate, where 

 cross-fertilization, for some reason, is impossible, the 

 individual thus isolated can store up its own spermatozoa 

 to be used in fertilizing its own eggs. But the fact 

 that this rarely happens is testimony enough that such 

 occurrences are inimical to well-being. 



The Lamellibranch, or bivalve Mollusca, e.g., Oyster, 

 Mussel, and Cockle, afford valuable evidence as to 

 excrescences and extravagances of growth which appeal 

 to our eyes as ornamental, and therefore likely to be due 

 to the influence of sexual selection. And this because 

 such ornamentation is a very conspicuous feature among 

 these animals. Yet, save in a few cases, locomotion is 

 impossible, and sight is wanting. Light-distinguishing 

 organs, and therefore eyes, are possessed by some, but 

 in no case probably are they strong enough to appreciate 

 form. Even if they did, such revelations of beauty would 

 play no part in mate selection from among the most 

 ornamental ; for these creatures are commonly fixed 

 throughout life in one position, often, indeed, buried 

 in mud or sand. Some move laboriously : a few, like the 

 Cockles and Pectens, swim by rapidly opening and closing 

 the shell. The Pectens are brilliantly coloured, not only 

 as regards the shell, which is also beautifully sculptured, 



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