TRANSFORMATION AND EQUIVALENCE OF FORCES. 219 



the unorganized contents of an egg into the organized chick, 

 is altogether a question of heat: withhold heat and the pro- 

 cess does not commence; supply heat and it goes on while 

 the temperature is maintained, but ceases when the egg is al- 

 lowed to cool. The developmental changes can be completed 

 only by keeping the temperature with tolerable constancy at 

 a definite height for a definite time ; that is — only by supply- 

 ing a definite quantity of heat. In the metamorphoses of 

 insects we may discern parallel facts. Experiments show 

 not only that the hatching of their eggs is determined by 

 temperature, but also that the evolution of the pupa into the 

 imago is similarly determined; and may be immensely ac- 

 celerated or retarded according as heat is artificially supplied 

 or withheld. It will suffice just to add that the germination 

 of plants presents like relations of cause and effect — relations 

 so similar that detail is superfluous. 



Thus then the various changes exhibited to us by the 

 organic creation, whether considered as a whole, or in its two 

 great divisions, or in its individual members, conform, so far 

 as we can ascertain, to the general principle. Where, as in 

 the transformation of an egg into a chick, we can investigate 

 the phenomena apart from all complications, we find that the 

 force manifested in the process of organisation, involves 

 expenditure of a pre-existing force. Where it is not, as 

 in the egg or the chrysalis, merely the change of a fixed 

 quantity of matter into a new shape, but where, as in the 

 growing plant or animal, we have an incorporation of mat- 

 ter existing outside, there is still a pre-existing external force 

 at the cost of which this incorporation is effected. And 

 where, as in the higher division of organisms, there remain 

 over and above the forces expended in organization, certain 

 surplus forces expended in movement, these too are indirect- 

 ly derived from this same pre-existing external force. 



§ 71. Even after all that has been said in the foregoing 

 part of this work, many will be alarmed by the assertion, 



