OF LIFE, AND ITS CONDITIONS. 35 



has reference not merely to its use as a material for the construction of the 

 fabric; food serves also as a generator of force; and this force maybe of 

 various kinds, Heat and Motor-power being the principal but by no means 

 the only modes under which it manifests itself. We shall now inquire what 

 there is peculiar in the sources of the Vital Force which animates the organ- 

 isms of the higher animals at different stages of Life. 



15. That the Developmental force which occasions the evolution of the 

 germ in the higher Vertebrata is reallv supplied by the heat to which the 

 ovum is subjected, may be regarded as a fact established beyond all question. 

 In the Frog and other Amphibia, which have no special means of imparting 

 a high temperature to their eggs, the rate of development (which in the 

 early stages can be readily determined with great exactness) is entirely gov- 

 erned by the degree of warmth to which the ovum is subjected. But in Ser- 

 pents 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 tem- 

 perature much above that of the surrounding air. 1 In Birds, the develop- 

 mental process can only be maintained by the steady application of external 

 warmth, and this to a degree much higher than that which is needed in the 

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

 tion as very significant of the dynamical relation between Heat and Devel- 

 opmental 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 

 attained 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 attained 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 development is 

 attained at the expense of a much greater consumption of nutrient material ; 

 the store laid up in the " food yolk" and " albumen" of the Bird's egg being 

 many times 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 evi- 

 dence 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 develop- 

 mental process, that the return of a portion of the organic substances pro- 

 vided for the sustenance of the embryo, to the condition of simple binary 

 compounds, is an essential condition of the process ; and since it can scarcely 

 be supposed that the object of this metamorphosis can be to furnish heat (an 

 ample supply of that force being afforded by the body of the parent), it 

 seems not unlikely that its purpose is to supply a force that concurs with 

 the heat received from without in maintaining the process of organization. 



16. The development of the embryo within the body, in the Mammalia, 

 imparts to it a steady temperature equivalent to that of the parent itself; 

 and in all save the implacental Orders of this class, that development is car- 

 ried still further than in Birds, the new-born Mammal being yet more com- 

 plete in all its parts, and its size bearing a larger proportion to that of its 

 parent, than even in Birds. It is doubtless owing in great part to the con- 

 stancy of the temperature to which the embryo is subjected, that its rate of 

 development (as shown by the fixed term of utero-gestation) is so uniform. 



1 In the Viper the eggs are usually retained within the oviduct until they are 

 hutched. In a Python, which went through the process of incubation in the Zoo- 

 logical Gardens, the eggs were imbedded in the coils of the body; the temperature 

 to which they were subjected (as ascertained by a thermometer placed in the midst 

 of them) averaging 90 F., whilst that of the cage averaged 60 F. 



