426 CORRELATION OF PHYSICAL AND VITAL FORCES. 



ness) is entirely governed by the degree of warmth to which 

 the ovum is subjected. But in serpents there is a peculiar 

 provision for supplying heat ; the female performing a kind 

 of incubation upon her eggs, and generating in her own body 

 a temperature much above that of the surrounding air.* In 

 birds, the developmental process can only be maintained by 

 the steady application of external warmth, and this to a de- 

 gree much higher than that which is needed in the case of 

 cold-blooded animals ; and we may notice two results of this 

 application as very significant of the dynamical relation be- 

 tween heat and developmental force first, that the period 

 required for the evolution of the germ into the mature embryo 

 is nearly constant, each species having a definite period of 

 incubation and second, that the grade of development at- 

 tained by the embryo before its emersion is relatively much 

 higher than it is in cold-blooded vertebrata generally; the 

 only instances in which any thing like the same stage is at- 

 tained without a special incubation, being those in which (as 

 in the turtle and crocodile) the eggs are hatched under the 

 influence of a high external temperature. This higher devel- 

 opment is attained at the expense of a much greater consump- 

 tion of nutrient material ; the store laid up in the " food yolk" 

 and " albumen" of the bird's egg, being many tunes greater 

 in proportion to the size of the animal which laid it, than that 

 contained in the whole egg of a frog or a fish. There is 

 evidence in that liberation of carbonic acid which has been 

 ascertained to go on in the egg (as in the germinating seed) 

 during the whole of the developmental process, that the return 

 of a portion of the organic substances provided for the suste- 



* In the Viper the eggs are usually retained within the oviduct until 

 they are hatched. In the Python, which recently went through the process 

 of incubation in the Zoological Gardens, the eggs were imbedded in the 

 coils of the body ; the temperature to which they were subjected (as ascer- 

 tained by a thermometer placed in the midst of them) averaging 90 F., 

 whilst that of the cage averaged 60 F. 



