=Periodicity in Breeding.=--In the marine Mollusca, the winter months appear to be the usual time for the deposition of eggs. Careful observations have been made on the Mollusca occurring at Naples,[248] and the general result seems to be that for all Orders alike the six winter months from November to April, roughly speaking, are the breeding time. Scarcely any forms appear to breed habitually in August, September, or October. On our own coasts, Nudibranchiata come in shore to deposit their ova from January to April. _Purpura lapillus_ may be observed depositing ova all the year round, but is most active from January to April. _Buccinum undatum_ breeds from October to May; _Littorina_ all the year round. The land Mollusca exhibit rather more periodicity than the marine. In temperate climates they breed exclusively in the summer months. In the tropics their periods are determined by the dry and rainy seasons, where such occur, otherwise they cohabit all the year round. According to Karl Semper, the snails of the warm Mediterranean region arrive at sexual maturity when they are six months old, _i.e._ before they are fully grown. After a rest of about three months during the heat of summer, a second period of ovipositing occurs.[249] _Helix hortensis_ and _H. nemoralis_ ascend trees, sometimes to a height of forty feet, when pairing.[250] =Hybridism= as the result of union between different species of Mollusca is exceedingly rare. Lecoq once noticed[251] on a wall at Anduze (Gard) as many as twenty specimens of _Pupa cinerea_ united with _Clausilia papillaris_. No offspring seem to have resulted from what the professor calls ‘this innocent error,’ for the wall was carefully scrutinised for a long time, and no hybrid forms were ever detected. The same observer noticed, in the Luxembourg garden at Paris, and M. Gassies has noticed[252] at various occasions, union between _Helix aspersa_ and _nemoralis_, _H. aspersa_ and _vermiculata_, between _Stenogyra decollata_ and a _Helix_ (sp. not mentioned), _H. variabilis_ and _pisana_, _H. nemoralis_ and _hortensis_. In the two latter cases a hybrid progeny was the result. It has been noticed that these unions generally took place when the air was in a very electric condition, and rain had fallen, or was about to fall, abundantly. Of marine species _Littorina rudis_ has been noticed[253] in union both with _L. obtusata_ and with _L. littorea_, but no definite facts are known as to the result of such unions. =Self-impregnation= (see p. 44).