202 THE POPULAR SCIENCE MONTHLY. 



whole earth's surface has determined the entire series of forms which 

 have existed in the past, and have survived till now. 



As we recede from the present into the past, it necessarily follows, 

 as a consequence of the ultimate failure of all evidence as to the con- 

 ditions of the past, that positive testimony of the conformity of the 

 facts with the principle of evolution gradually diminishes, and at 

 length ceases. In the same way positive evidence of the continuity 

 of action of all the physical forces of Nature eventually fails. But 

 inasmuch as the evidence, so far as it can be procured, sui3ports the 

 belief in this continuity of action, and as we have no experience of 

 the contrary being possible, the only justifiable conclusion is, that 

 the production of life must have been going on as we now know it, 

 without any intermission, from the time of its first appearance on the 

 earth. 



These considerations manifestly afibrd no sort of clew to the origin 

 of life. They only serve to take us back to a very remote ej^och, 

 when the living creatures differed greatly in detail from those of the 

 present time, but had such resemblances to them as to justify the con- 

 clusion that the essence of life then was the same as now; and through 

 that epoch into an unknown anterior period, during which the possi- 

 bility of life, as we understand it, began, and from which has emerged, 

 in a way that we cannot comprehend, matter with its properties, 

 bound together by what we call the elementary physical forces. 

 There seems to be no foundation in any observed fact for suggesting 

 that the wonderful property which we call life appertains to the com- 

 binations of elementary substances in association with which it is 

 exclusively found, otherwise than as all other properties appertain to 

 the particular forms or combinations of matter with which they are 

 associated. It is no more possible to say how originated or operates 

 the tendency of some sorts of matter to take the form of vapors, or 

 fluids, or solid bodies, in all their various shapes, or for the various 

 sorts of matter to attract one another or combine, than it is to explain 

 the origin in cei'tain forms of matter of the property we call life, or 

 the mode of its action. For the present, at least, we must be content 

 to accept such facts as the foundation of positive knowledge, and from 

 them to rise to the apprehension of the means by which Nature has 

 reached its present state, and is advancing into an unknown future. 



These conceptions of the relations of animal and vegetable forms 

 to the earth in its successive stages lead to views of the significance 

 of type (i. e., the general system of structure running through various 

 groups of organized beings) very different from those under which it 

 was held to be an indication of some occult power directing the suc- 

 cessive appearance of living creatures on the earth. In the light of 

 evolution, type is nothing more than the direction given to the actual 

 development of life by the forces that controlled the course of the 

 successive generations leading from the past to the present. There 



