SCATOPHAGA LUTARIA AND S. STERCORARIA. 415 



descendants of that individual, such as might be brought about 

 by an exceptional mortality of the unmodified relatives, or that 

 with the modification occurred a correlated alteration in the 

 genitalia (and these parts are without doubt exceedingly variable), 

 the favourable modification would be swamped before it had time 

 to establish itself. Lord Tennyson, speaking of Nature, says : * 



" That I considering everywhere 



Her secret meaning in her deeds, 

 And finding that of fifty seeds 

 She often brings but one to bear." 



Fifty, as applied to species, would be an economy wholly different 

 from Nature's methods. For one modifi.cation that has established 

 itself as a species, an enormous number, only to be guessed at by a 

 calculation of the laws of chance, have appeared and disappeared. 



In the case of those flies that lay their eggs in the tissues of 

 leaves or plants, and have developed a horny ovipositor for this 

 purpose, a character is found that, first occurring in the female, 

 has gradually modified the other sex, and finally produced such 

 very specialised genitalia as those of Palloptera ustulata — an 

 exceedingly complicated process of evolution. Here, from the 

 very nature of the case, the variation would have less danger of 

 being " swamped," and probably this circumstance has had an 

 important influence on the successful establishment of these very 

 pronounced characters. However, the families whose females 

 bear a horny ovipositor are few in number compared with the 

 ordinary type. Four families with 135 species are given in 

 Mr. Yerrall's list of British Diptera, a list which catalogues 

 over three thousand species. 



To study genitalia it will be necessary to dissect under the 

 microscope. A freshly killed insect is best ; but old dry specimens 

 are not altogether useless, as by soaking them for about two 

 hours in a 10 per cent, solution of caustic potash, followed by an 

 hour in water, the soft parts expand and the chitin ceases to ba 

 brittle. 



A compressorium or a live box is useful after the organs have 

 been "teased" apart, which must be done in water contained in 

 a watchglass or shallow cell. Add a drop of glycerine, compress^ 

 and examine with powers of about ^rd and ^th inch. An interesting 



* *' In Memoriam," Iv. 



