PERIODICITY. 1 35 



winter, would probably die in the winter if its growth during 

 the summer were checked by any cause. Thus a climate that 

 varies between two remote extremes of temperature must neces- 

 sarily give rise to a sharply defined periodicity, by excluding ] 

 those forms which have by nature a somewhat prolonged period 

 of individual growth, since these, as the conditions requisite for 

 their development are not fully satisfied, must have to contend 

 with the temperature which checks their growth. 



The opposite state of things must prevail in equable, and 

 above all in tropical climates, whese the variations in tempera- 

 ture are reduced to a minimum ; here the periodicity of animal 

 life must be to a great extent obliterated, at any rate in so far 

 as it depends on temperature. For every individual of a species, 

 even if its period of individual growth is longer or shorter than 

 the average of the species, can live and multiply if it is never 

 exposed to an absolutely fatal degree of heat ; according to the 

 laws of inheritance, its progeny will probably have a tendency 

 to develope more or less slowly, and so by degrees one individual, 

 whose period of individual development is short, may produce, 

 let us say, six generations in a year, while another of the same 

 species, but of slower growth, may produce only four. In this 

 way all periodicity, as regards summer and winter, must be 

 entirely lost, and at last fully grown individuals and young ones, 

 larvae and freshly laid eggs will all be found together at every 

 season and in every month of the year. Such cases, in fact, are 

 not uncommon, although they have been but little heeded 

 hitherto. Nothing in the Philippine Islands struck me so much 

 as to observe that there all true periodicity had disappeared even 

 from insects, land molluscs, and other land animals ; I could at 

 all times find eggs, larvae, and propagating individuals, in winter 

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

 certain periodicity, which is chiefly perceptible by the reduced 

 number of individuals in the dry months and the greater numbei 

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

 eggs are hatched under great drought than when the air is veiy 

 moist. Even in January, the coldest and driest month, I found 

 land snails which require much moisture, and at every stage of 

 their development, but only in shady spots, in woods, or by the 



