178 NATURE AND MAN. 



it which exerts itself in building up and maintaining a certain 

 structural type. Thus each species puts to a use of its own the 

 heat that is supplied to it; just as, if we may use so rough a 

 simile, each of the machines in a large manufactory may turn out 

 a particular kind of work, although the same motor force is sup- 

 plied to all ; and each generation transmits to its successor, not 

 the force, but the capacity, for making a particular use of the 

 force; just as a machine would do, that could apply its motor 

 power to the construction of another machine similar to itself. 



The study of the life-history of cold-blooded animals — those, 

 namely, whose temperature closely follows that of the medium 

 they inhabit — leads to precisely the same conclusions ; as is 

 especially apparent in those cases in which the ratio of life can be 

 most accurately estimated. The earliest developmental changes 

 in the fertilized egg of the frog, for example, consist in the cleavage, 

 or segmentation of the yolk-mass, first into two parts, tlien into 

 four, then into eight, and so on ; and it was found by Mr. Newport 

 that the periods at which the successive cleavages took place were 

 so precisely determined by the temperature to which the eggs were 

 exposed, that he could predicate the former from the latter with 

 great precision. So it has long been known that the production of 

 larvcE from the eggs of insects could be accelerated or retarded, 

 like the germination of plants, by increase or diminution of tem- 

 perature ; and that the same holds good also regarding the pro- 

 duction of the perfect chrysalis in the last metamorphosis. In the 

 adult animal, the rate of life may be in some degree estimated by 

 the amount of carbonic acid thrown off in respiration ; and it has 

 been shown by the experiments of Dr. W, F. Edwards, that this 

 increases in a direct ratio to the temperature to which the body is 

 exposed ; whilst the duration of life, when respiration is prevented, 

 is much greater at low temperatures than at high, showing that the 

 animals then live much more slowly. 



The case is different, however, with warm-blooded animals ; for 

 they are rendered in a great degree independent of external varia- 

 tions, by the power which they possess of generating such an 

 amount of heat within themselves, as shall keep the temperature 

 of their bodies up to a certain fixed standard. Hence it is that 

 their rate of life varies very little, and that their developmental 



