THE BREEDING SEASON 13 



Among the land-Mollusca there is a more marked periodicity in 

 the breeding season than among the marine forms. In temperate 

 climates breeding is restricted to the summer. In the tropics the 

 occurrence of the breeding season is generally determined by the 

 alternations of wet and dry seasons. In other cases, where there are 

 no great seasonal changes, the land-Mollusca may breed all the year 

 round. 1 The snails of the Mediterranean area, according to Semper, 

 arrive at sexual maturity when they are six months old, and before 

 they are fully grown. Those individuals which reach this age in the 

 spring deposit eggs a second time after the heat of the summer is 

 over, and so experience two breeding seasons in the year, with an 

 interval of a few months between them during the hot weather. 

 Semper shows, further, that individuals of the same genera, or 

 perhaps even of the very same species, in the damper and colder 

 climates of the north, do not lay eggs till development is complete ; 

 while in the dry, warm region of the Mediterranean, they have 

 produced two lots of eggs before they are fully grown. This is 

 because completion of growth and sexual maturity do not necessarily 

 coincide. In a similar way, in the pond-snail (I/imncea) the 

 minimum of temperature which admits of the assimilation of food, 

 and so of growth, is much above the winter temperature of egg- 

 deposition. 



In tropical climates, where the variation in temperature through- 

 out the year is reduced to a minimum, the periodicity in the breeding 

 habits of animals is to a considerable extent obliterated, at least in so 

 far as it is dependent upon temperature. Semper 2 says that few 

 things impressed him more in the Philippine Islands than the 

 absence of all true periodicity in the breeding habits not only of the 

 land-molluscs, but also of the insects and other land-animals. "I 

 could at all times find eggs, larva;, and propagating individuals, in 

 winter as well as in summer. It is true that drought occasions a 

 certain periodicity, which is chiefly perceptible by the reduced 

 number of individuals in the dry months, and the greater number in 

 the wet ones ; it would seem that a much smaller number of eggs 

 are hatched under great drought than when the air is very moist. 

 Even in January, the coldest and driest month, I found land-snails 

 which require much moisture, and at every stage of their develop- 

 ment, but only in shady spots, in woods, or by the banks of streams. 

 But what was far more striking in these islands was the total 

 absence of all periodicity in the life of the sea-animals, particularly 

 the invertebrata ; and among these I could not detect a single species 

 of which I could not at all seasons find fully grown specimens, young 

 ones, and freshly deposited eggs." Semper goes on to remark that 



1 Semper, loc. cit. 2 Semper, loc. tit. 



