AGASSIZ AND EVOLUTION: 19 



The obstacle in the way of the acceptance of the derivative origin 

 of species was the then prevalent notion concerning the nature of life. 

 We must briefly sketch the change which has taken place in the last 

 forty years in our ideas on this subject. 



Until about forty years ago, the different forces of Nature, such 

 as gravity, electricity, magnetism, light, heat, chemical affinity, etc., 

 were supposed to be entirely distinct. The realm of Nature was 

 divided up into a number of distinct and independent principalities, 

 each subject to its own sovereign force and ruled by its own petty 

 laws. About that time it began to be evident, and is now universally 

 acknowledged, that all these forces are but different /bnns of one, uni- 

 versal, omnipresent energy, and are transmutable into one another 

 back and forth without loss. This is the doctrine of correlation of 

 forces and conservation of energy, one of the grandest ideas of mod- 

 ern times. But one force seemed still to be an exception. Life-force 

 was still believed to be a peculiar, mysterious principle or entity, stand- 

 ing above other forces and subordinating them ; not correlated with, 

 not transmutable unto, nor derivable from, other and lower forces, and 

 therefore in some sense supernatural. Now, if this be true of living 

 forces, it is perfectly natural, yea, almost necessary, to believe that 

 living forms are wholly different from other forms in their origin. 

 New forms of dead matter may be derived, but new living forms are 

 underived. Other new forms come by natural process, new organic 

 forms by supernatural process. The conclusion was almost unavoida- 

 ble. But soon vital force also yielded to the general law of correlation 

 of natural forces. Vital forces are also transmutable into and derivable 

 from physical and chemical forces. Sun-force, falling on the green 

 leaves of plants, is absorbed and converted into vital force, disappears 

 as light to reappear as life. The amount of life-force generated is 

 measured by the amount of light extinguished. The same is true of 

 animal life. As in the steam-engine the locomotive energy is derived 

 from the fuel consumed and measured by its amount, so in the animal 

 body the animal heat and animal force are derived from and measured 

 by the food and tissue consumed by combustion. Thus, vital force 

 may be regarded as so much force withdrawn from the general fund 

 of chemical and physical forces, to be again refunded without loss at 

 death. This obstacle is, therefore, now removed. If vital force falls 

 in the same category as other natural forces, there is no reason why 

 living forms should not fall into the same category in this regard as 

 other natural forms. If new forms of dead matter are derived from 

 old forms by modification, according to physical laws, there is no rea- 

 son why new living forms should not also be derived from old forms 

 by modification according to physiological laws. Thus, at last, the 

 obstacle was removed — the ground was cleared. 



But Science is not content with removal of a priori objections. 

 She must also have positive proofs. The ground must not only be 



