86 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



dence of the same kind in regard to the influence of Heat upon the pro- 

 cesses of nutrition and development. Thus the time of emersion of 

 Insect-larvae from their eggs, or in other words, the rate at which the 

 previous formative processes go on, is entirely dependent upon the tem- 

 perature. In the case of the Bird, we find that, if the temperature be 

 not sufficient to develope the egg, chemical changes soon take place, 

 which involve the loss of its vitality ; or if the temperature be reduced, 

 so low as to prevent the occurrence of those changes, the loss of heat 

 is in itself destructive of life. But this is not the case in regard to the 

 eggs of cold-blooded animals in general ; for, like the beings they are 

 destined to produce, they may be reduced to a state of complete inac- 

 tion by a depression of the external temperature ; whilst a slight eleva- 

 tion of this renews their vital operations, at a rate corresponding to the 

 warmth supplied. Hence the production of larvae from the eggs of 

 Insects may be accelerated or retarded at pleasure ; and this is, in fact, 

 practised in the rearing of Silk-worms, in order to adapt the time of 

 their emersion from the egg to the supply of food which is ready for 

 them. The same may be said in regard to the eggs of other cold- 

 blooded animals ; those, for example, of the minute Entomostracous 

 Crustacea (Water-Fleas, &c.), which people our ditches and ponds. In 

 many of these, the race is continued solely by the eggs, which remain 

 dormant through the winter ; all the parents being destroyed by the cold. 

 The common Daphnia pulex produces two kinds of eggs ; from one, the 

 young are very speedily hatched ; but the others, which are produced in 

 the autumn, and enveloped in a peculiar covering, do not give birth to 

 the contained young until the succeeding spring. They may be at any 

 time hatched, however, by artificial warmth. 



125. We sometimes find special provisions for imparting to the eggs 

 a temperature beyond that which is natural to the bodies of the parents ; 

 thus it has been shown that in Serpents, the temperature of the poste- 

 rior part of the body rises considerably, when the eggs are lying in the 

 oviduct, preparatory to being discharged, evidencing a special heat- 

 producing power in the surrounding parts at this period, which is obvi- 

 ously for the purpose of aiding the maturity of the eggs. The Viper, 

 whose eggs are frequently hatched in the maternal oviduct, so that the 

 young are brought forth alive,^is occasionally seen basking in the sun, 

 in such a position as to receive its strongest heat on the parts that cover 

 the oviduct. Certain Birds have recourse to substitutes for the usual 

 method of incubation. The Tallegalla of New Holland is directed by 

 its remarkable instinct, not to sit upon its eggs, but to bring them to 

 maturity by depositing them in a sort of hot-bed, which it constructs of 

 decaying vegetable matter. The Ostrich is believed to sit upon its eggs, 

 when the temperature falls below a certain standard, but to leave them 

 to the influence of the solar heat when this is sufficient to bring them 

 to maturity; and this statement derives confirmation from a similar 

 fact observed in a Fly-catcher, which built in a hot-house during several 

 successive years, the bird quitting its eggs when the temperature was 

 high, and resuming its place when it fell. In all these cases, as in 

 many more which might be enumerated, we observe the influence of an 

 elevated temperature upon the processes of development ; and the pro- 



