Permutations of Sex 



If, however, he Is not yet convinced, here is 

 something to remove his last doubts. 



The Three-horned Osmia often settles her 

 family in old shells, especially those of the 

 Common Snail {Helix uspersa) , who is so 

 common under the stone-heaps and in the 

 crevices of the little unmortared walls that 

 support our terraces. In this species, the 

 spiral is wide open, so that the Osmia, pene- 

 trating as far down as the helical passage per- 

 mits, finds, immediately above the point which 

 is too narrow to pass, the space necessary for 

 the cell of a female. This cell is succeeded 

 by others, wider still, always for females, ar- 

 ranged in a line in the same way as in a 

 straight tube. In the last whorl of the spiral, 

 the diameter would be too great for a single 

 row. Then longitudinal partitions are added 

 to the transverse partitions, the whole result- 

 ing in cells of unequal dimensions in which 

 males predominate, mixed with a few females 

 in the lower storeys. The sequence of the 

 sexes is therefore what it would be in a 

 straight tube and especially in a tube with a 

 wide bore, where the partitioning is compli- 

 cated by subdivisions on the same level. A 

 single Snail-shcU contains room for some six 



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